I expect that the astral is present in all of those places, but is unknown and/or inaccessible. That would include the MtG settings.
That is my view too. Wherever anything exists, wherever a conscious mind exists, the Astral Plane exists. The peculiarities, like alignment planes, ethereal plane, elemental planes, fey and shadow, might not exist in other settings − these planes of the "Wheel" might be a particular interconnecting system within one region of the Astral Plane. But the Astral Plane includes anything and anything imaginable. One can travel via the Astral Plane to the Magic The Gathering settings, for example, even if they arent part of the Greyhawk/Realms Wheel cosmology.
I have fun with the skyey elves.
That they are optional for settings is correct. However, it's not really accurate that it's optional in the book.
The book itself says the species are optional. So they are optional.
You and I have no ability to decide whether it's in the PHB or not. It's inclusion or exclusion is non-optional. Whether we decide to use half-elves or not doesn't affect whether half-elves are in the game(different from setting). If they are in the PHB, they are in the game no matter what we want for your personal games/settings. The same goes if they are excluded.
Settings may or may not have Genasi, or may have an other version of Tiefling.
The Aasimar in the game are an "option", which may or may not exist in a setting. Their being in the 2024 Players Handbook has no special "canonical authority" except for convenience. The default is the multiverse including any setting.
I haven't seen that language. Someone in this thread did quote language that did not say that, but rather they assumed that the language meant that players could go back and pick from 5e.
Can you quote the passage that explicitly says that so that I can see it.
For example, in page 177 of the Players Handbook.
"
ORIGIN COMPONENTS
Each part of your characters origin reflects facets of your character, their life, and the circumstances that started them on the path to adventure. If you choose a background or a species from an older book, see the sidebar "Backgrounds and Species from Older Books" in chapter 2 for how to use them with the options here.
"
The 2024 species are options, and species from other books are also options. A particular campaign might use a setting that picks all, some, or none of them. There is no "default setting", except for the "multiverse", which includes any setting.
A kitchen sink setting, like Forgotten Realms, probably will have every option in play, including the options from the "older books". But this "completist" approach is itself a setting conceit, and has no special canonical authority over any other setting.
The 2024 rules require a character to have a "species", as a part of the "origin". But what that species is might be anything. Whatever the table wants. There is advice for how to make it work.
Not to a setting, but to the rules, yes there are. Every race in the 5.5e PHB is a default race for the 5.5e game.
The rules dont privilege any particular species, except by making ten species convenient to pick, when deciding on a species.