You should be able to look at the html output by viewing page source. Near the top you will probably see some extra text output that is likely error messages and is what is actually causing the header information message. That information may be helpful in finding out exactly where the error is coming from. (But it is possible that it is only the actual “Warning” message itself that is causing the other header messages.)
But that error is coming from WP internal code, but it could be caused by a plugin that uses the built in WP function.
So far, I don’t know of any other PHP 8 issues with the theme.