Low magic does not reside in the items or the power of some casters but in the availability of high level caster. An 8th level wizard is not high level by standard and historical definition. Name level was the bare minimum to be considered "high" level. This means that in 1ed a wizard had to be 11th level. In the old Greyhawk appendice, 10% of the population was classed. Of these:
From the 1st edition box set of Greyhawk about High level NPC and Random encounters.
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This means that in a castle + village 500, you might see 50 classed NPC. Very few of these might qualify as high level. Perhaps one maybe two? Of these 50% of the time it would be a fighter and 60% chance that this NPC might not even be higher than 12th level. All the rest would be below named level, that is 9th level for fighters, 10th for thieves and so on. This is not what we can see in Eberron or in FR. The occurence of high level characters is staggering compared to what we see in Greyhawk. I could even show the page of High level important NPCs and the list is quite short. 1 page for a whole continent!
The rarity of high level casters was two folds.
1) The requirements for being a caster. The minimum ability to cast a spell was directly related to the stat. There was even spell failure if you had a wisdom stat lower than 13. The minimum to cast spell was 9 but it also meant that you would never cast spell of 5th level and higher as the minimum stat requirement for a spell of level 5 was 10. Then the table would diverge wheter intelligence or wisdom was the stat. With no ASI available, you were stuck with what you had. Without the maximum stat (18) to cast the highest level spells, it meant that your caster might be gimped into not ever be able to cast these spells ever. With no upcast possible, it litteraly meant that some of your spell slots would never be of use, ever. This is one of the reasons many casters would retire at some time. They would not be able to cope with the power creep.
2) Training was a thing. Finding a higher level trainer to train faster and cheaper was also a thing. Otherwise the cost of training between level was astronomical. With access to money and a good trainer, it meant that some casters were effectively stuck at their current level if they could not find the funds to train. This was happening often enough for people to consider retiring. Players were often sharing gold in a manner that would allow everyone to train but it was not all groups.
Today, it would be difficult to emulate these factors. The only way to do it would be by going the gritty way. 5ed can be relatively easy but if you apply the optional rules in the DMG for resting and a few others, it can be quite deadly. But would it be enough? I doubt it. A 5ed Greyhawk would be filled with more higher level NPCs than a 1ed one.
