I ran into this today. I already had the extension and had been using it without issue, but it seemingly disappeared overnight. Quitting Safari, reinstalling the extension, and rebooting the computer all did not help.
I remembered reading something similar a little while ago and dug it up: https://lapcatsoftware.com/articles/disappearing-safari.html
I ran this in Terminal.app, as the article suggested:
/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -f -R /Applications/Safari.app
and it fixed the issue for me! Hopefully this helps anyone else who starts seeing this problem :smile: