But not as any sort of official category. WotC grouped them up in 3e, but there was never any need to do so. A half-demon isn't the same race as a half-celestial, which isn't the same race as a half-elemental. Neither are their descendants.
I agree there was never any need to restate the category, but just because it wasn't stated doesn't mean it doesn't exist. After all, the same races are in it for the same reasons, and none of that has changed.
Additionally, is a half-human the same race as a half-human who is the same race as a half-human? You seem to want to focus on one half, while completely ignoring that it takes two halves to make a whole.
So do gnomes, elves, dwarves, firbolg, tritons, and probably some others.
Different names and different resistances. Other races also get those things. They don't have a unified resistance in common like a race would.
The one and only racial ability they have in common is darkvision.
Gnomes, Elves and Firbolgs get spellcasting, but do not get elemental resistances. (exceptions for the elves on the New Astral Elves [tied to the Astral plane] and the Shadar-Kai [Tied with the Shadowfell])
Dwarves get elemental resistance, but not spellcasting (except Duergar who have a history of being affected by the lower planes)
Tritons get spellcasting and elemental resistance... and their entire niche is protecting gateways to the Elemental plane of Water, giving them a connection to one of the planes.
So, it seems like my evidence is holding fairly strong. Both traits together seem to be indicative of a relation to a plane, like being touched by that plane.
I have no idea as I don't have the new book, but even if that's the case, it's irrelevant here. All the "planetouched" races were made prior to these new rules.
So evidence of a thing being done isn't evidence that it might have been considered before.
They are not one race. They are like giants, dragons and other categories which have multiple races within them. Think of them like Homo Sapiens and Neanderthals. Related, but different races.
And yet they are different races.
So, what is a race then?
Because you want to claim that elves are a single race, with subraces that include vastly different abilities, appearance, and even one of them has gills and lives underwater. Yet in the same breath you want to say that Giants are multiple different races that have no subraces. So why is it I can have a Wood Elf, a Sea Elf and a Shadar-Kai and them all be "elves" but having a Frost Giant, Cloud Giant and Storm Giant is somehow all being different races?
What makes the difference here? What is a race? Until you define this I'm left just looking at these examples and saying that you are wrong and don't know what you are talking about, because you have two seemingly identical situations and you are treating them differently with claims of special pleading.
And still your major hurdle is that Genasi, Aasimar and Tieflings are all separate races in 5e. Do you have any hard fact that show otherwise?
Why do you claim they are separate races? I understand and accept that they have different mechanics, but you have never once demonstrated how that makes them separate races and not just different subraces.
You continually refuse to define the terms, and thus your claims appear to be special pleading with no substance behind them.
Descendants. It says descendants, not children. And we have cambions as the children of demons and mortals. You know, half-demon cambions.
You think "fiendish blood" proves that they are children? Nope. Doesn't prove that at all.
Sure, I guess it could come from a blood transfusion, but generally children carry the blood of their parents.
Additionally, your children are ALSO your descendants. That's how words work. The cambions are more powerful tielflings, I won't deny that, but much how you have Changelings and Doppelgangers, they seem quite clearly to be the same basic type of being.