It sometimes feels like restrictions are speed limits, they're made to be broken.
If you have a world without gnomes, someone will want a gnome. If you have a world with all the PHB races, someone will want one from another book. If you have all the offical races someone will want a 3PP one. If you have a bunch of 3PP ones someone will want a custom one....
I had a game where the set-up was that everyone was supposed to be a non-demi-human and they would have a ring that let them appear as a demi-human for several hours a day. One wanted to be a human who was cursed to be a humanoid and the ring made them look like their real self... (I mean it was fine and worked well, but it feels like folks will always push beyond whatever limit was set).