I know it seems strange but there’s a regulatory and safety reason for the ashtrays.
If the lav door was designed and approved with the ash tray, it must continue to be installed and maintained. The airlines can’t just remove parts even if they seem to be insignificant without approval and certification. We had a door flying around at my airline without an ashtray and got fined for it…And the door didn’t have a provision for it.
There continues to be knotheads who light up in the lavs. Ever since the Air Canada incident in Ohio where a a guy tried to hide a cigarette butt in the trash can and set the aircraft on fire inflight, it’s felt that there should be some tested, certified and safe place to extinguish just in case.