They should disclose at booking time ideally, just as they should disclose anything which might affect the booking, e.g. PE or ED.
My best bookings have been with clients who have a disability or medical issues who've told me upfront about it and asked if I can accommodate their needs. If I can't then I always say (e.g. some hotels I use don't have lift access so aren't suitable for a client who can't do stairs.) Usually it's just a case that they are looking for someone non-judgmental and patient and who will let them lead the booking rather than a pre-conceived plan of kiss, undress, suck, fuck, cum, get out.
I understand that some clients may feel embarrassed or be afraid of being turned down, but they are shooting themselves in the foot as they're likely to end up booking someone who won't be prepared for their needs, and they'll have a shit time.