Apparently there is a dev(?)-accessible switch on a per-site basis that disables the script that cleans out tags that have only been used once in 6 months. Arqade recently turned this off because they had a problem with questions losing game-name tags. We have a smaller problem with this, in losing system tags.
The disadvantage of turning this off is having to manually prune the tags list of underused tags. Arqade has accepted this tradeoff, given they've got many eyes on the problem. I think we should accept this tradeoff, given that we don't get new tags added that often and losing system tags causes more work and consternation (especially when a tag wiki is lost) than would the infrequent tags that needs pruning.