I am running my test forums on both php8 and php8.1, browsing, posting and attaching is working fine, the control panel did not report any errors in the server logs, but i am not sure if there might be hidden issues I am not aware of
The XF devs still don't rate the latest 228p1 as fully compatible with PHP 8.1 and don't recommend using it, so yours is gonna be a whole lot worse, even if it seems ok to you during testing.
This begs the question of why don't you just update to the latest XF version? There's literally hundreds of bug fixes and improvements in it. I use 228p1 with the latest PHP 8.0.x and it works beautifully.