Comment 2 for bug 1761136

dovecot-pop3d: /usr/lib/dovecot/pop3

This binary is part of the package.
That it is gone breaks your current config.

But this should be no normal state, even installing and removing dovecot-pop3d will not break it that way, so I'd assume something like a manual rm or by a script got you in this state.
You'd have to fix up your config to upgrade it properly.