PHP 8.0 was release only 7 months ago - We still haven’t run full tests on PHP 8, so I would recommend keeping the MainWP server on PHP 7.x
PHP 8 is still pretty new and we will need more time to run tests before we can confirm full compatibility.
You may follow this thread here on the topic as well: PHP 8 Not work with MainWP - #6 by josklever