What is the current process/policy for auto-deletion of single question tags?

Question

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.

Answer 1

Short, simplified, practical answer

On any main site that has not opted out of automated tag cleanup, any tag that has gone more than six months with only a single question will be deleted unless it has at least a wiki excerpt defining it. The time period is extended to 12 months on meta sites, but still works the same.

Long answer

The gist of the automated single-tag deletion, and the thinking behind it, is described in two Meta Stack Exchange threads.

• Should We Zap The Low-Occurrence Tags? describes the initial rollout of automated single-question tag removal.
• Do not expire single-use tags that have a tag wiki describes adding a provision to exclude those tags for which a tag wiki has been defined.

This cleanup routine applies to tags used on only a single question for more than six months on main sites, or more than 12 months on meta sites (including Meta Stack Exchange), which don't have a defined wiki. The usage history is irrelevant; it includes tags created for one question and never used on another, and tags used at one time on multiple questions and then reduced to a single question (because it was removed from questions, or tagged questions were deleted). The cleanup is run monthly, so tags might not be deleted immediately upon qualifying.

Tags with zero questions (orphan tags) are deleted by a different cleanup routine which runs on a daily schedule, with two exceptions:

• If the tag is a synonym of another tag, and is removed from all questions using it, it will leave a zero-question "stub" so the system can continue to recognize it.
• Certain default tags will always continue to exist on per-site metas (child meta sites).

Technically, the exclusion for tags with a defined wiki requires both the wiki excerpt and the wiki. However, the system creates an empty placeholder for both when either is created, whether it's by a user with tag wiki editing privileges (trusted users) or when a user without that privilege submits a proposed wiki or excerpt as a suggested edit. The cleanup routine recognizes even an empty placeholder as existing, so as a practical matter, creating either satisfies the requirement.

This operational quirk can lead to edge cases. For example, as merely submitting a suggested wiki or excerpt creates an empty placeholder for both, if (both) the proposal(s) is/are rejected, it will leave both empty placeholders, and the tag cleanup process will incorrectly recognize that as a tag to retain.

There are pros and cons of automated tag cleanup, largely discussed in the above threads. On large sites, it can be a big labor saver. However, on some sites, applying the cleanup rules would regularly delete desirable tags, creating more problems than benefits. For example, on Movies & TV, some movies or shows may only have one question asked about them at a time, but that doesn't mean tags about those movies or shows aren't good fits for the site. As such, that site has opted out.