If you go back to the march 2017 update it does say that the costume function is available *for now*. It wasn’t planned on being permanent, it was just a fun little thing to play with in the early days of the game. the early days are over and we’re getting closer to the business days.
I agree that the smarter move would be to allow the early donors to get all the costumes they had access to for free before newer costumes come out. Or even just the costumes that they actually used if that data is available. At the Very least, the costume they had on when the update rolled out.