Discovered this on #252415 but thought it'd be better off on a PR of its own.
It looks like support for vpopmail is being dropped in dovecot 
I have not been able to identify its deprecation in the documentation  or changelogs, but as seen in #252415 some files related to vpopmail have started already.
I had missed this in 2.3.13 release notes:
* auth: Removed passdb-sia, passdb-vpopmail and userdb-vpopmail.