Show me where in the Forgotten Realms I can find Animism, Dualism, Monotheism and a single tight pantheon. Those, like the vast majority of examples, are NOT Forgotten Realms examples.
Yes. Page 38 when it talks about flavors of fantasy. Low Magic is an option mentioned.
I mean specifically all elf? No. Does it tell you that you can pick and choose which races are in the world? Yes. Yes it does. page 26 it tells the DM to decide any on restrictions and options, including new or prohibited races. Prohibited races would be all by elf if the DM wanted to pick that.
Meh. If you read the entire DMG this is not hard to come up with.
You seem to be under the misapprehension that the game has to tell the DM about every possible option. It doesn't. It just has to include enough different options to get the DM thinking about his having the ability to change things. The 5e DMG does that in spades, so the DM can easily come up with that idea and enact it.
All that you listed are things the DM can easily come up with by reading the DMG and getting his creative juices flowing. There's no need for every possible world build scenario to be presented, and in fact no RPG book ever printed could possibly do so. It would take at least several thousand pages.
Again it talks about the idea. It doesn't teach.
Here's a quick example of what I mean.
"Races in your campaign.
Choosing The spread of races in your campaign setting is a major aspect of worldbuilding. Which intelligent species are in your world and which ones are available for players to play as creates the overall feel of your campaign. You should choose races that match the setting you decide to make and you are comfortable with.
Races in the PHB are typically seen as standard and available to players in most campaigns. You asked the DM have to final say of which races are available in your setting. However the restriction of races in the PHP should be carefully thought about. The spread of races chosen in the PHP was decided to give a wide array of character archetypes in phenotypical forms. Removal of one or two of these races might remove a kind of character which is very common in play. For example removing Orc and Goliath removes the "big man" trope from play. You may still remove them but it is a good idea to add back in the feeling of playing a big man in some other way. Perhaps allowing a custom feet to a human to allow them to be big and have access to the Powerful Build
Races, themes, styles and genres
You may also curate your races based on the theme
You may not want to include a very inhuman race like Trikeen as playable if your campaign has a theme about humanity fighting the weird of the wilderness.
In a swashbuckling campaign you might not want to include a race that has a swim speed or is able to breathe underwater like the Sea elf or Triton in order to keep the danger of drowning at sea always in the player's heads
If your campaign has a low Magic style you might not want to include elves or tieflings or aasimar who have natural magic.
The Rarity Tool
You can use rarity as a way to include races and species that do not completely match your style of setting. By making them rare but part of the world you can include them without warping the logic behind that setting. Perhaps the tritons control underwater empires and their rarity above sea level makes them a spectacle on swashbuckling boats. Perhaps your elf is the only elf within 20 mi as the elves do not share their magic with the wide community of non-magical intelligent beings.
Example
You might include all of the Players Handbook species except for the Goliath. In their stead you include the Powerful Feat and Runecaster feat seen below.
Orc might be available but orcs have a revulsion to seaborne travel so the played orc would be one of the few orcs anyone in the campaign ever sees and will be treated as such. No Orc NPCs should be expected and Orcish will be mostly a useless language to know.. Any stereotypical behavior and stories will be attributed to the player character orc.
For the player who desires to have an aquatic species, You include the sea elf and the Triton as playable options. You tell the player that the Sea Elf Kingdom and the Triton Empire have been at war for thousands of years and by picking either of these races, you have automatically chosen to have great animosity with the other side as well as the enemy of both major underwater powers the kraken cult who have ties to the Far Realm. In addition the player will now have to come up with a reason why their character is not underwater fighting the war under the sea and is instead above the oceans."