There is an automatic job that deletes single-use tags after 6 months. No attempt is made at determining whether the tags are good and should remain, or whether the tags are a variant name of another tag and should be renamed or made synonyms. No attempt is made to retain tags that have proved their worth, for example with a tag wiki. There is no way to explicitly declare a tag as good. Even beta sites, where it is expected that the tag system is still evolving, are not exempt.. This job is not subject to any review, not even after the fact: one day the tags are there, the next day they're gone.
This has been shown to cause harm on Security. We currently have three untagged questions. What were they tagged as? No way to know. Untagged questions are only the tip of the iceberg: there is no way to know that a question had a tag deleted if it still has a non-deleted tag.
- turn off the job that deletes single-use tag, and provide a log of past deleted tags so that we can add them back where appropriate; or
- provide evidence that the deletions were beneficial to the site (I don't know what form the evidence could take other than a log of past deleted tags so that we can ascertain that the tags were indeed useless in a vast majority of cases).