As I have learned from here and here, tags which have no questions are deleted once in 24 hours and tags which have been only used once are deleted after 6 months.
I would like to know whether such pruning is used at metas, too, and whether the frequency is different there.
Since the amount of questions at metas is much smaller than at the main sites, maybe even tag with single occurrence might be useful there.
I already know that there is one difference between the pruning on main sites and metas. The default tags which were imported from meta.SO to all metas are not deleted, even if there are no questions tagged, as I've seen here.