I feel it will be handled like prophets now with upcoming patch notes-- just don’t take the mick with soecifics. Keep things vague, and of course make room for error.
I think intentional metagaming will be a smaller problem than unintentional metagaming. We know a lot now, and that’s not easy to forget. Tauren couldn’t be priests and sunwalkers were unheard of, so a character who casually mentions such is making an opsie. Same goes for trolls chilling with gonk and being somewhat druidic. Where are people gonna draw the line on what was then and what is now? Because I think if you start involving a bit of “now” I’m classic, such as these cultural examples where we’re told “yeah, they’ve been a thing, we just didn’t know about it” back in classic when it literally was unknown, you’re creating real blurred boundaries and problems. How did your troll get this druidic teaching? From whom? Where are they? So better to just push the erase button and forget all about troll druids in classic and any predilection they may have for druidic magic.
It’s this kind of stuff I envisage will cause issues. It won’t be done with malice, but it’s that stuff that’s informed by a knowledge of what happens later, to create a character before that happened.