That’s true. Just keep in mind that it’s no zammad issue per se. The libapache2-mod-auth-kerb package simply isn’t released for bullseye (yet). We ran into that issue with our monitoring software too.
If you know what you're doing
You’re perfectly able to backport the sid package though (Debian -- Details of package libapache2-mod-auth-kerb in sid), at least that’s what we did. As soon as the bullseye package is released one can go back to the stable repo for this.
A small hint in the zammad docs could prevent some confusion i guess. Feel free to create a PR for that.
cheers