To stop the warnings, configuration would need to be changed in PHP config, or those warnings be silenced by the developer.
Any decent webapp has had this issues resolved a long time ago.
I would suggest that websites that will be effected probably will not migrate to PHP 8 that quickly.
But there are a million questions on the internet about this 'error'. And the answer is almost always: turn the errors off.
And I believe this is the biggest problem with PHP. A lot of inexperienced users with questions that get very bad answers.
For a development and staging website you do want notices and warnings displayed. For a production website possibly only when testing something.
Just SSH in and tail the log. Dealing with unexpected output from errors clobbering your headers isn't worth the trouble.
You can also try something like FirePHP [0] if you need the errors in the context of the current page.