Actually finding and quoting the language from the ADA (the different versions and the supporting documents) is difficult. In the past it was somewhat possible, when Google allowed you to do a search send then keep narrowing down the search results by adding words to search within the results. You could have initially several hundred thousand results and finally get down to several hundred results by adding words to search within results.
Most of the searches now just point toward a huge document and good luck finding the words in it you are looking for. I don't have time to do that, but have looked in the past and did find the things I am going to mention.
There is language in various places in the ADA and that prohibits requiring documentation of disability for public accommodations (this includes WDW and other amusement parks) - a sentence here and a sentence there.
Some of the places include accessible seating, using a wheelchair, needing an accessible room. There are other places, like the Service Dog parts, which say prof can't be required, but specify questions that can be asked.
Documentation can be required for work related (for example, if your job requires lifting and you need a limit lower than what is required). Really, for work, any accommodation that requires the workplace to spend money, change job requirements/responsibilities or work process. .
And, for either work related or public accommodation (which is where WDW falls), there are clauses that say the person with a disability doesn't have to share the nature of their disability, but does need to be able to share what their needs related to their disability are. I can't quote the specific language, but it amounts to 'if you won't tell us what your needs are, we don't have an obligation to accommodate them'.
There is some language about
being able to require documentation or (I believe the words used are 'other credible assurance') of disability if what is provided would be something others would pay for or would be considered of value. That is where the handicapped parking permits, disability transportation, discounts or things like providing free access to Universal or 6 Flags 'Fastpass-like' service.
There is also language that talks about 'full and equal enjoyment' , which some take to mean "whatever I feel would give me full and equal enjoyment is required to be provided."
It has never been as absolute as that - those words butt up against the parts of the ADA that talk about "reasonable accommodation" and detail that the public accommodation is not required to change the nature of the experience or make large changes in their regular operation/procedures. The simplest to see is wheelchair accessibility - my daughter's 'full and equal enjoyment' would require every attraction to accommodate a wheelchair without transfer, but that is not required (or even expected).
The 'full and equal enjoyment' is the part that the people suing Disney about
DAS appear to be primarily basing their claim - i.e. That they require unlimited access to whatever attraction they want without waiting in order to get 'full and equal enjoyment'.
That was part of the tactics a few years ago that people using Segways as mobility devices used; they were not successful because Disney was able to prove in court that Segways would constitute a danger in a crowded theme park and there were other means they could use instead.
Full and equal enjoyment to someone with food allergies might mean they feel they need separately prepared meals, but the ADA information about that says the accommodation is doing simple things like omitting an ingredient, like a sauce, etc. It is also allowable to pass the cost along to the consumer if a product (like gluten free checking tenders) are more expensive. The restaurants can choose to do more ( and most do), but are not required to.