It appears like phpBB Debug (i.e. warnings printed to screen) is turned on automatically in a php 8 environment, even when debugging is turned off.
Steps to reproduce: Upgrade from php 7.4 (no debugging messages) to php 8.0.3 (debugging messages).
Once in php 8.0.3, debugging can be turned off again by uncommenting line 105 in common.php
#set_error_handler(defined('PHPBB_MSG_HANDLER') ? PHPBB_MSG_HANDLER : 'msg_handler');