Not exactly.
First, it's in the rules. I don't think this particularly needs a PSA, any more than "Hey, you can play as a human" does. But then again, we've been playing with custom backgrounds since the beginning, and I guess I should not be shocked, at this point, that people fail to RTFM. Even five years in. ¯\(ツ)/¯
The pushback in this thread, IMO, started because of the usual divide between those who insist something must be in the game, and those who believe that the RAW are just a starting point for the table to play. It's kind of a divide that happens fairly often.
Again, I have yet to play a game of D&D where I haven't discussed my character with other people before starting play, so this whole, "You get to do what you want, and no one, I MEAN NO ONE INCLUDING THE DM CAN STOP YOU" is really foreign to me. YMMV.