A main thing is that people with autism would just be classified as generally mentally disabled and the rise in autism is highly tied a drop in that general diagnosis. I don't think that covers 100% of the rise but does seem to make up the big majority.
U.S. special-education autism classification was created in 1994 and tied to a big rise in diagnosis.
https://news.wisc.edu/data-provides-misleading-picture-of-au...
Yes, this absolutely. You can't study something after altering it.
The "treatments" for people with any kind of neurodivergence (real, or imagined) in the past were often interventions that destroyed enough of their brain or body to prevent them from exhibiting any neurodivergent symptoms (e.g. lobotomy, EST/ECT, teeth-pulling[1], etc).
[1]: https://www.amusingplanet.com/2019/01/henry-cotton-psychiatr...