As T266583: MediaWiki 1.35 should be compatible with PHP 8 per PHP support policy isn't complete (nor for master) and T269710: MW 1.35 Library Bumps for PHP 8.0 very much so isn't either..
We should probably add notes to the MW documentation that while 1.35 is destined to support PHP 8.0, there are potentially various issues that need addressing still. Please test and report issues etc