As long as it's a race or creature allowed in their character creation, then it's okay. But a High Elf Wizard (Bladesinger) that is turned into a Half Orc is out of luck until the True Polymorph is dispelled.
That's not the way I read the FAQ guidance. Specifically, look at where it talks about Reincarnate. Being reincarnated into some other race doesn't preclude you from progressing in classes you had before, so I don't see why True Polymorph would.
Here's what it says...
Dragons Aren’t Allowed in Town?!
A character that ends an adventure as a race or creature not otherwise available through their character creation rules is removed from play until the effects of the true polymorph spell are dispelled.
Permanence Isn’t Just a Number.
A true polymorph upon which concentration has been maintained for the entirety of its duration becomes a permanent spell. As a permanent spell, it can still be dispelled and its effects removed.
So a Bladesinger (Elf Only) ends an adventure as a Half-Orc. It is then removed from play until the effects of True Polymorph are dispelled. How else do you interpret this?
There's a difference between 'asking on Facebook' and something in the official FAQ. As a DM I'll listen to what a player tells me was 'said on Facebook' but my discretion would be final. However the FAQ is canon. Is there something in the FAQ I missed about Reincarnate?
FAQ said:What if I get Reincarnated?
If your character is killed and subsequently reincarnated, the DM must roll on the table provided in the reincarnate spell’s description—they may not choose the race you return as. You may continue to advance in whatever classes you possessed normally; your new race doesn’t preclude you from advancing in an archetype you already possess.
So a Bladesinger (Elf Only) ends an adventure as a Half-Orc. It is then removed from play until the effects of True Polymorph are dispelled. How else do you interpret this?
All I can think of is that the True Poly rule is to keep players from continuing to advance as a Greater Devil or the like while Reincarnate is already bounded by permissible permutations.