I haven't read the entire thread so apologies if this has already been said.
I think the percentage of ND is probably the same, but (a) we are more aware of it now and (b) current lifestyles trigger it more, whereas it might have gone under the radar more easily.
My reasons for saying this are:-
My son is ND, he was far better as a small child on "down days" i.e. playing at home with his toys rather than being out at soft play etc. too many days out were definitely an issue.
So my typical childhood day was good for him. He definitely struggled with expectation that kids are constantly taken out rather than allowed to stay home and just play with toys.
At school, especially secondary, it is so geared to academic achievement. All the hands on practical stuff geared towards kids less academic has gone. When I was at school the less academic were often out and about on local farms etc., learning practical skills.
The job market is totally different, a lot of office work is based around computers, geared to inputting info correctly. Large companies has post rooms, messengers, tea ladies, filing clerks, a plethora of openings that would give someone an "in" to a company to allow them progress at their speed. That's all gone.
It was hard to find a job for my ND son. He has found his niche and, at the moment, is ticking by. It's not well paid, but it's a job.
There are other examples, but you get the jist. Life was less stressful and more manageable in years gone by, so a lot of ND kids and adults could find their niche, tick on by, and just went under the radar.