This would probably be a controversial take, but it's 11 pages in so not as many people will see it to tell me how evil I am on this.
Autism does not necessarily mean you are smarter or better at something, but it can mean that it may be harder for someone to express it socially or for others to see it in a social situation if someone is extremely smart and/or brilliant.
It is a different way of seeing or understanding social cues and situations from what I have seen at times.
I am one of those who feel that numerical representations perhaps do a better job of showing something than someone trying to act something out.
For example, if someone has asthma, it will affect their ability to do certain things. It doesn't mean they can't overcome such things, but it will make it more difficult. So, a -2 (depending on how bad their asthma is, if it is REALLY bad to the point it could kill them, perhaps a -4 or more even) to their Constitution would make it appropriate I think.
So, someone could feasibly start off with a Constitution of 16 if they wanted to really spend an 18 on it. Now, with 5e one can actually add points to their ability scores, so someone who really wanted to focus and increase their Constitution could eventually even have a 20 Constitution if they REALLY wanted it. It is harder for them to get their as they start off with a disadvantage in that arena, but it IS possible for them to overcome this and be better than many others!
The REASON I do this is that it can be HARD to play someone with a disability and remember it. This is even harder if you have never had it, but want to experience what it may be like to have to deal with something like that and/or overcome it or become better at what it affects than others. A simple modifier makes it so that you are at a disadvantage with the areas it affects in a way that is subtle enough that it affects everything that you do with that ability score, but at the same time you don't have to account for it constantly in a way that actually may not truly be how it affects someone with that ability (for example, I've known those who have asthma who have become cross country runners, marathon runners, and more. However, they still had challenges to get to that point, and at times people who didn't have to worry about using inhalers or other medications may not realize what those challenges were).
In that same light, I'd probably give someone who wanted to play a character with Autism a -2 (or -4 or more for extremely challenging cases) to their Charisma. Charisma is more of a social ability and from that come many of our social skills in D&D. Someone with Autism could still be quite Charismatic, but it will be tougher for them to over come other things which some who do not have the difficulties of social interactions may face.
This reflects the difficulties one may have in facing the challenges of social interactions that some may have with this. It's not necessarily a handicap, but a different way of dealing with social cues and interactions.
Probably something others will hate on me for, but I think this would be my solution to the question. It's simple and yet does what it is set out to accomplish. It's a reflection of a difficulty one may face, but not something that cannot be overcome or even be an area one can excel at if they so choose to focus on that area.