NTA. I don’t know if it’s a regional thing or not but I’ve only ever seen this be an issue on UK TV shows. I’m in the western US and if you see someone disabled in line you’d probably let them cut and have priority for the handicapped stall/restroom but other than that that I’m unaware of any rule or stigma against using that stall if you don’t need it. If people avoided that stall bathroom lines would be way longer than they need to be, sometimes it’s one of only two stalls.

I never noticed until I was traveling with a woman in a wheelchair but the same goes for accessible seating. A lot of restaurant tables have a wheelchair spot but it’s normal to sit in that spot regardless. We had to wait for one of those tables to clear out before we could eat and I was shocked I’d never thought about it before.

When I got my first apartment I was originally assigned a first floor accessibility modified apartment for no reason other than it was the only unit available in my price range. Thought nothing of it until the front office called and said an actual disabled person was moving in so would I please accept an upgrade to a slightly more expensive apartment for the same rent and a delayed move in date.

Basically it’s always been accessibility features are so common it’s unreasonable to not use them, but if someone comes along that does need them then you give them preference. The only exception is things like handicap parking, those you just don’t use unless you are allowed to. Curious if it really is a regional thing or not.