Current documentation should state that pubring.gpg is not used by default anymore
and only used under some circumstances.
The place where it needs updating is
The behaviour is already described starting here
https://dev.gnupg.org/source/gnupg/browse/master/doc/whats-new-in-2.1.txt;94d31660c6db22c3b539f440994d286f687c273f$640
what the whats-new-in-2.1.txt is not directly available from the main webpage.
Only for reference:
This report was triggered by a question about what the files do
https://wald.intevation.org/forum/forum.php?thread_id=2194&forum_id=21&group_id=11