It looks like each state does things slightly differently; the state itself has to "obey" the 504 rules but they can have their own overlapping ones "instead" if they want as long as everyone gets covered. Here in Maryland they do IEP's (Individual Education Program) which are state-related and 504 plans only come in if they still can't participate in their classes well. For example, someone in a wheelchair might not need an IEP because they're only physically handicapped but might qualify for help under 504. Sounds like New York just uses the 504 to guide for everything?

I was in school in the 1950s and 60s before they had any of that. They mostly tried to "mainstream" who they could and all the other kids were in special schools "out of sight". Most kids with emotional or psychiatric problems weren't separately identified, it was just treated like we had behavioral problems unless they were really bad and then that kid would be sent away.