Xenforo supports 8.1, they just don't recommend it yet.
Really... is that why certain developers and support have been VERY concise that 8.1 was not a supported version under the 2.2.1x script?
There are a WEALTH of posts here that are directly related to 8.1 and issues it has with the current version of XF... and MANY of them posted in the bug fix area.
Like I said, last I looked, XF developers did NOT recommend you run 8.1 as it was "cutting edge" and the current version of XF was not "certified" for PHP 8.1 compatibility... but you do you and enjoy living on that cutting edge. Will it run.. yep... will it have issues that will crop up, yep, as proven by the bug fix entries.
Realize.. we aren't talking the "cutting edge" of PHP having been out a while.. but the cutting edge of the fact that it has not been reliably & extensively TESTED against the current versions of the script. That's generally referred to either ALPHA or BETA testing.. and has not been (at least since my OS/2 2.1 days) been recommended as a position of stability - and I BETA'd MANY an OS/2 app related to FidoNet back then (actually ran 2.0 for a while until 2.1 came out, and prior to that DESQview for a multi-node BBS).