When I first wrote the script I erroneously started from the assumption that
pagure.io_users == src.fp.o_users
However, that assumption is wrong: while it is true that all pagure.io users must have a Fedora account, not all packagers have their user set in pagure.io. Therefore tagging an inactive packager here in pagure.io doesn't guarantee that we reach out them.

To avoid removing users that are simply inactive but still reachable we need to find an alternative way.
In the next run the tickets where the user is not registered in pagure.io will be highlighted by an specific tag. However, that means someone will have to manually process those tickets by sending out an email.

The easiest way to overcome this problem would be to have an empty repository set on src.fp.o with bug ticket management active and open tickets there, so that we are sure that tags will work as expected. But I'm not sure a) it's possible to have such repository and b) it's possible to enable bug ticket management on src.fp.o.

Another option would be to set up a mail system in the script and have it send emails directly.