I’m having the same problem, Bitwarden extension vanished in Safari 15.0 (Mac OS 11.6.1).
Seems to be a recurring problem, so there are several fixes for several versions of Safari and Bitwarden. Tried most if not all, including rebooting, reinstalling Bitwarden, installing Safari Technology Preview, running OnyX to rebuild Launch Services database. No luck. As a cherry on top, restoring old version of Bitwarden from Time Machine brings back the extension but desktop app refuses to log in (with a “unknown error” or smthng).
Looking to replace Bitwarden with something more reliable.