Last time we went to HDDR, DD10 had a broken ankle and required a wheelchair because she wasn't allowed putting weight on her foot. We were in category 3. I had a note attached to my ADR stating that we needed accessible seating. I was told that there are tables at the back on the ground floor for folks in category 3 who need accessible seating. I was also told that even though the note is on my ADR that it would be a good idea to show up early to make sure that there were enough accessible tables to accomodate us. In the end we allowed DD10 to decide if she could hobble up the stairs or not and she chose to go up so we didn't need the accessible seating. Looking at the number of scooters waiting, that was probably a good thing since there were quite a few (we were the first to check in so we would have gotten a table but since DD10 COULD get up the stairs it seemed like the right thing to do to go up).
Hopefully they're keeping track of how many ADRs that are slated for upstairs require the accessible seating so they'll actually know at what point they're sold out.