I will ask someone more knowledgeable than me about this when I have a chance, but I think it became common to look at the severe cohort under the label of "profound autism," a few years ago, but I think has a tendency to be listed as a % of overall ASD cases rather than a separate rate.
But also noted that they find a lot of the research lacking - they view research, even that using the "profound" label, as not doing a sufficient job of mapping severity bands: the DSM now has three levels of severity, but research going back decades is hard to map to the evolving buckets. And they view the profound label as probably too narrow for fully encompassing kids who needs high levels of support in the educational setting.
Friend also says the way they would probably look at it in their school district is by label and support level: ASD, and then whether they need para pro support, or require a self contained classroom (IE a specialized classroom environment), and in terms of the ASD labels needing para pro or self contained setting going up in quantity even while district overall enrollment declines. And those support levels pretty much guarantee that either kiddo has severe behaviors or parents have lawyers, and the vast majority of parents don't have lawyers.
They attribute some of this to non-exceptional-education kids being drawn off to charter schools (which despite theoretical obligations basically do a good job of not providing real services for special needs kids to get them back to publics rather than reducing profits), but not remotely all of it.
Separately, one of my relatives has kiddos with what I'd colloquially call severe autism (like I don't see their kids ever living independently) and their district tried to move a big chunk of their elementary high support kids to a middle school because they "ran out of space" for elementary self contained / autism classrooms.
