I was getting this error: PHP Deprecated: Use of Sanitizer::removeHTMLtags was deprecated in MediaWiki 1.38. [Called from SimpleTooltipParserFunction::infoTooltip in [redacted]/extensions/SimpleTooltip/modules/SimpleTooltipParserFunction.php at line 61] in [redacted]/includes/debug/MWDebug.php on line 377
This is because MediaWiki deprecated the removeHTMLtags. I created a fork of this at GitHub. If you go to Github dot com and search for fanra/simpletooltip you can find it (links not allowed here). The version I created does not give that error. It's currently version 2.0.0.
It works fine for me but use at your own risk, I am disclaiming any responsibility for any possible issues.