We have tried PHP 8.0 and 8.1. Both will work, but after we ran performance tests on one of the large UNA communities (4+ million members) with an intensive calculation (feed calculating in runtime) we found that 7.4 is a little faster than 8.0 and significantly faster than 8.1. It's not clear why, but we decided to table this investigation until 8.2 is stable and we can run tests with all of them more extensively.