"Turn display_errors off" is a good answer for a production website. You will get problems with headers and cookies, if notices are printed before headers are sent. You might also divulge information about your stack to users.
For a development and staging website you do want notices and warnings displayed. For a production website possibly only when testing something.
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.