In the early days of SE, there was discussion about auto-deleting low-usage tags. The pro argument was that they didn't serve a useful tag function and diluting tag topics made tagging less effective. The con argument was that good tags that might gain popularity later and might serve a niche audience now would be deleted.
In 2010, Jeff Atwood reported that he "implemented a routine that removes tags created more than 6 months ago (12 months on Meta sites) which have been used only a single time." Several years ago, I was advised that tags with only one question for more than six months (either no more added after creation, or reduced to one through question deletion), were automatically deleted (a variation of the original description). A quick search of Meta questions didn't spot any recent ones discussing a change to this process.
I noticed some single-question tags on Arts & Crafts. They are mostly very old and I don't recall seeing those tags on any questions in the last few years. There is virtually no deletion of old questions, except for Roomba, and it's rare for that to delete ancient questions. This suggests that these have been single-question tags far longer than six months.
If these tags are, indeed, fodder for automatic deletion, some of them may be worth protecting. That would entail some work and an investment of time. We would also want to alert tag creators of the need to ensure that their new tags get at least a second question. But none of that is necessary if these tags are not in danger of auto-deletion.
- What process/policy is currently in place for auto-deletion of single-question tags, and does it make a difference how the tags became single-question (never used on another question vs. multi-question use, then question deletion down to single)?
- Does the process include any variation by site or site characteristics in the application of automated tag cleanup (e.g., not applied to sites below a certain size because of the difficulty of filling out questions for tags expected to be more used in the future)?
- Does the process include a provision for "marking" a tag to protect it from auto-deletion (other than adding another question), or a site opting out of automated tag cleanup?
Update - In the comments, An exception is described for tags that have a "wiki", and there is a link to the associated original discussion on Meta. The [status-completed] tag on that discussion indicates that this exception was implemented. In all of that discussion, "wiki" is used imprecisely.
The tag pages refer to the abbreviated blurb as "tag wiki excerpt" or "usage guidance" and the body of the description as the "wiki". If you look at a tag page with only an excerpt, it says:
"There is no tag wiki for this tag … yet!".
Jeff Atwood's response about implementing the change refers to requiring the tag wiki to be completed (emphasis mine). That suggests entries in both the excerpt and the wiki.
Rummaging through the comments on that linked thread, there is discussion about the implications for Bible Hermeneutics, which was a small beta site at the time. So that addresses whether the process affects all sites.
So the remaining question is whether the wiki requirement is satisfied by just an excerpt. The comments suggest that that's people's understanding. The existence of at least a few old single-question tags on A&C with just an excerpt suggests that that's the correct interpretation. Absent a definitive answer, I'll assume that's how it works.
But it would be useful to users who are willing to donate their time to curate site content, who were not here in 2012, to have ready access to a definitive answer.