This is really ignorant. That’s not an insult exactly, I mean that you’re asking questions that all have answers if you cared to look them up.

The reason why people call that sort of attitude “ableist” is that you’re requiring me, the disabled person, to prove that I’m not a liar.

> What specific accommodation is being asked for?

There is a whole field of study called “Disability Theory” that includes this topic. It is well-defined with thousands of pages written on the topic. It is not nebulous, you just aren’t familiar with it.

> What counts as a disability?

Words have definitions:

“the condition of being unable to perform as a consequence of physical or mental unfitness”

> Am I supposed to take someone's word that they have ODD

Yes, but a person with ODD is still expected to be a decent human. They may make more often mistakes but they should recognize and apologize for those mistakes. They should be in therapy to help learn self-regulation techniques.

Likewise, you should give them some extra grace because, in spite of all that extremely hard work, it’s hard to ignore what your body is telling you (push back against authority) and sometimes they will slip up.

> can I request proof from a medical professional?

I dunno, can I get a video of your last colonoscopy? Can you tell the public internet if you take take viagra? Can you list your mental health disorders for me please? How’s your gut, had any diarrhea lately?

Medical information is private.

> There are real tradeoffs here.

Yes, the tradeoffs are that things are a bit less convenient for able-bodied people and disabled people get to actually have agency on their lives.