Including RTL-LTR flips, character substitutions etc? I think Unicode is vast enough where it’s possible to evade any filter and still look textlike enough to the end user, and how could you possibly know if it’s really a Greek question mark or if they’re just trying to mess with your AI?

I assume that anyone trying to "filter" the text could just render it and then OCR it.

Ultimately the AI will just learn those tokens are basically the same thing. You'll just be reducing the learning rate by some (probably tiny) amount.