Having clicked options before reading the first post (not always advisable, I know. But these things are usually fairly self-explanatory) I had to go back, because clicking 6 wasn't an option, and then read the post and just hit "I'll explain."
First, I thought the caster options were referring to "low/medium/high" degrees of power (got that one right! ) but then the "frequency" (since the title seems to indicate we're asking about the setting) was in reference to the "frequency," i.e., "number of casters" to be found in the setting of said power level.
So, I'm still not 100% on what the options are trying to get at...like you can use "low/medium/high" level magic but can only cast it with "low (not often/rarely - or rare per day?)/medium (per RAW spell progression?)/high (a lot/often)"? ... or that I think the spell progression is too slow (low)/medium (it's fine RAW)/too fast-too powerful (high)? ... or is it that the caster's "power in the setting" is low/medium/high...that is, a 7th level caster still has access to 4/3/2/1 spell slots...they just don't/can't do it a lot/often...or there are so many uber archmages running around that a 7th level caster is barely on the radar of the truly powerful...or something else...and thus the "caster" is considered a "low powered" for the setting?
I also had no idea (before reading the post) that the "Setting" options were intended for "magic items, creatures, locations" but thought it was more a general "access" and commonality of magic to be found in the world...but not specifically "items/locations/creatures." SO, that in mind kind of changes some calculus...
The most comprehensive/overall answer I can come up with is: "My homebrewed setting is not a monoculture."
So, pretty much all of this variable. There are places you can buy some potions or scrolls or low level magic items. I would say, this is basically "most" places. It's just for ease of play...definitely when I begin a campaign, you'll be starting in a place/region where this is possible.
But, then, there are areas where no one could possibly do that...and there would be very few, if any, persons/creatures to make them in the first place.
There are places rife with "everyday" magic, heating/cooling foods, changing colors of clothes, keeping things clean, lighting the town lamps at dusk, etc... but things like "fireballs" or "banishing demons" or (for sure) "teleporting" or "raising the dead" is not close to something people even think of as possible.
There are places where if there isn't a teleportal circle or flying ship to take you where you want to go, your average person is going to think, "What kind of backwoods slackjaw slum is this?!"
There are also all of the variables/various regions where "clerical magic" will be more prolific than wizards and mages or druids. Regions where druids are the magic workers and anyone else are either "false prophets" (clerics of the pantheon/Gods of Men) or some kind of demon worshipper/in league with dark powers (clerics or mages).
And then are, of course, the regions where the scholarship of wizardly sagely pursuits reign supreme...druids and clerics looked down upon as "weaker" because it is not their "own" power. While psychics, on the other hand are to be feared/mastered/studied and/or snubbed out for their bizarre "interior magic" that rivals or -gasp- could surpass the mage nobility's hold on power.
In all of these places, there are areas -some known, some secret/sacred, some completely unknown- that hold locations of varying power. Same for creatures. Sure, most people would know stories about dragons and griffons, giants and merpeople -some may even be accepted as infallible truth- but they have still never actually seen one...nor is it likely they would in their lifetime/region. There are other areas -well known to most travelers, so as to be completely avoided- where you can't throw a goblin without hitting a giant.
On a related note, the appearance and existence of "true" dragons has been in decline for some centuries, now. It is a matter of great concern to the species of real dragons (chromatic and metallic, the gems are already reduced to singular individuals), as their broods produce fewer and fewer fully/properly formed dragons. Instead, there is an influx of semi-draconic and dragon-like creatures (wyverns, hydrae, dragonnes, linnorms, etc...), if viable offspring hatch at all, being produced by the full wyrms. Naturally, this is something not shared with the mortal species...What good would it do? What could such limited momentary creatures possibly do to help a dragon?
There are the sacred druidic holdings: groves, grottos, henges, mountain tops, etc... that hold ancient deep powerful (but not necessarily flashy) magic. There are sites of holy power: where some demigod ascended or some holy champion defeated some unholy horror, or a "miracle" was performed (some cleric Raised Dead on some hero, banished some great demon, or some such), etc...
And there are known (and unknown) places where the planes may bleed into the setting or at least a portal is possible: faerie rings, ancient dolmens or other portals to Faerie or the Land of Shadow, elemental portals as can be found beyond the clouds atop Mt. Alhannan or in the Burning Sands, one of the Seven Deserts (the majority of which comprise the "Empire" of Thel). Full of volcanic activity, flame geysers, and the deadly firewinds, the region is largely controlled by a clan of Fire Giants, Efreet, and legions of Salmander troops/underlings that flood in from the Elemental Plane of Fire...but thankfully can not leave/extend much beyond the region's borders because it gets uncomfortably "cool" for them (as any Thelitian merchant will tell you, this is only "by the great grace of the Supreme Emissary, the Shadarsha," immortal ruler of the Thel).
So there are magic spots, and critters, and magic items of various stripes are produced in various regions.
Can you buy a Wand of Magic Missiles in yon farming hamlet? Probably not. Are they "Buy 2 get 1 free!" at "Selizor's Sorceries" on High Market Street in Flin (City of Sages and Scrolls), in the exotic high magic empire of R'Hath? Yes, you probably can (...though any respectable R'Hathi mage or Flin resident will warn you of Selizor's shoddy workmanship).
Can you get a clerical scroll (a few 1st or 2nd level spells) or healing potions from the small shrine down the road or cloistered abbey outside of town? Possibly...even probably.
Can you find a Ring of Shooting Stars or Staff of Power or a Dragonslayer Sword in a metropolitan magic shop? Unlikely. Outside of R'Hath, perhaps even impossible. The Dwarves of Naradun, however, if you can gain their trust and cooperation (and pay their price), can probably help you with that magic sword.
There's really no hard and fast answer for the entirety of the setting for any of these elements...spellcasters and spells cast included. Some areas are less magical (in all way, not just locations). Some places more. Some places MUCH more or MUCH less.
Some places where magic is rare, the magical elements there are are not particularly powerful. Some places that it is rare, there is a single overflowing font of amazing power with a single caster-guardian/keeper/protector...who has to kill you now that you found it.
In some places, magic is so prevalent, that someone casting a lightning bolt from within their globe of invulnerability at a flying four-headed basilisk would get a golf clap.
That's my setting. Where the party goes... What the players are really interested in playing... How powerful they want the magic power levels (which, clearly will be open/achieveable for their PCs) to be... that's all a case-by-case, game-by-game basis. The Setting can handle/present any of it.
I chose medium strength high frequency and medium power very common. The magic is very abundant but has limitations and I tend toward trying to provide balance while maintaining fun. In my campaign setting, the world is deadly and full of magic. Literally, everyone possesses the potential for magic but using it can have unwanted results if you're not of significant enough potential energy this can be augmented by consuming certain substances or by making use of magic in its physical form. My setting is not 100% typical high fantasy, however. There is magic-tech and the advances that naturally coincide with using physical magic as a power source for technology. Not to go into far too much detail but this has been limited due to an event in the past that resulted in terrible antimagic and wild magic storms to essentially hold the world hostage to some extent. I find that my setting as it is currently designed allows for more rewarding gameplay experiences through the ability for players to have access to a greater variety of magic and magic-powered items to use in their games beyond the RAW design philosophy of 5e and magic items. I feel this gives gold value again as well as offers motivation to seek out and explore "dungeons" beyond the class-level power treadmill. There is a lot more that I could say about this and maybe one day I will but for now, that's all I think is needed to answer the question.
The poll results are interesting and about what I would have expected to be honest. It appears we see a wide variety of campaign settings and dm styles present. With concentrations in the medium ranges which basically just show what I would have predicted, being that people tend to lean toward "averages".
Highly dependent on which setting I'm using, but when I DM in the Forgotten Realms, I take the "magic dial" and turn it 3-4 notches down. This leaves me with a setting where spellcasters are few but the ones that are there are very powerful and influential. Most are rivals and locked in a cold war; if one makes a move, so will their enemies.
In Eberron, it's the exact opposite. You can purchase magic toothbrushes at your local store, but you'd be hard pressed to find someone that can cast spells above 3rd level.
Oh, man. There is no wrong answer. I love low magic, high magic, medium magic, black magic, white magic, hedge magic, high magic, darksun magic, dragonlance magic, eberon magic, ritual magic, psionic magic, arcane magic, divine magic, bardic magic... well, maybe not bardic magic. JUST STOP MAKING ME DM!
The poll is worded well, but I'm not sure where the current game I'm running falls.
Homebrew setting, centered around a failing Imperium that used to have magi-tech when it was at it's peak, and there are still remnants around. Strong enought hat they created three races, and magiclly genocided another. But magic comes from the earth itself, which is the dead body of a decapitated god (it's skull the moon) and is a limited resource. Magic has been used up to the point that spells above 6th level just won't work, and highly magical creatures are either extinct (like dragons) or only exist near independent sources of magic (like unicorns near portals to the feywild).
But low level magic is still everywhere. Naval navigation has been by magical means for so long that they are just discovering celestial navigation now. Magical oaths of loyalty to the Imperium are common for all of the highest ranked military and other important positions.
There's been rather few magical items the entire campaign outside potions and scrolls. BUT the hook of the campaign is that each of the characters awoke one of the semi-sentient artifact Masks of the Imperium. There used to be hundreds, now the wearabouts of less than a hundred are known and most can "sleep" dormant for generations between finding a suitable bearer who is compatible. So they have a permanent item that grows with them.
(They also were rarified agents of the crown, so can requisition basically any mundane equipment as needed. Treasure has not been a big motivator for this group at all.)
So common low level magic but near non-existent high level magic. Very few items but starting with one that grows as the characters do. Casters are not rare, but ones able to cast higher than 2nd level spells are. Spells oif levels 7-9 literally cannot be cast without a separate source of magic, none of which are portable.
My settings have gods, spirits and fae interacting with a low magic world, so while I think DnD spellcasters get too powerful for the worlds I want modelled that doesnt mean the setting doesnt have wonderous things happen.
PCs however get that wonder via boons or roleplayed interactions with NPCs not from spell level