I am running Safari 14.0.3 on Big Sur (latest updates) and Bitwarden from the App store wouldn’t show up in the extensions prefs in Safari
I downloaded and installed [\Release Version 1.24.7 (Frozen) · bitwarden/desktop at Release Version 1.24.7 (Frozen) · bitwarden/desktop · GitHub
This works! Yay! Until they add official Safari support, this may be the way to go.