This was also our situation a few months ago.
Most projects were done and kept on the BIM Server but it just became a mess of different versions of Archicad (running multiple versions of BIM Server is a no no) and it just didn't look and feel organized.
So with the move over to AC20 I migrated everything up to 20 (5 years of projects)
and kicked off all the non-active projects of the BIM Server by saving them as a PLN and archive file.
This way the project gets saved with all library items intact and everything is saved safe and sound on our file server & cloud backup.
The migrating and saving part was a gargantuan pain in the butt but it was bad housekeeping on our side as well, I think there is a guide or post on the sort of best practices to archive projects, Ill post it when I find it
Andre Steynberg
Win 10 64-bit
AC27