5:11 AM
@MartinSleziak I see that the tag no longer exists. Since I see nothing in the revision history, I assume it was removed by an automated process, simply because the tag had only one occurrence.
Another possibility is that it was merged by a moderators - AFAICT this is silent, too.
If this information is up to date, the single occurrence tag with no tag-wiki has to be 6 months old to be removed automatically: Should We Zap The Low-Occurrence Tags?
Speaking about this, I see that Asaf Karagila created tag here: “Towers” on singular cardinals with countable cofinality.
8

Let $\lambda$ be a singular cardinal of countable cofinality. Is there necessarily a sequence $\{A_\alpha\mid\alpha<\lambda^+\}$ of countable subsets of $\lambda$, such that $\alpha<\beta$ if and only if $A_\alpha\setminus A_\beta$ is finite? In other words, we know that for $\omega$, there is ...

The tags with only one occurrence are removed unless they have a tag-wiki: Do not expire single-use tags that have a tag wiki. And, as I said above, the automated process looks at tags old at least 6 months.
Perhaps it would be useful to create some basic tag-info so that tag is preserved?
Personally, I would be able at most copy a few things from Wikipedia article PCF theory.
Is this tag actually different from another tag . And if yes - and it is worth preserving - would you be willing to create at least some tag-excerpt/tag-info?
Asaf Karagila: I left a few comments regarding the (pcf) tag in the MO editors' lounge. (Mainly the question whether it is different from (pcf-theory) and how it could be prevented from automatic removal.) Feel free to ping me there after you see the comment - so that I know that I can remove it. — Martin Sleziak 34 secs ago
Since this comment is unrelated to the question, I guess it's better to clean up a bit after it is no longer needed.
I see that the tag (biography) was created in this question. Tags with only one occurrence are removed after 6 months unless they have tag-wiki. So perhaps you could create some basic tag-info if you think that this tag is worth keeping. (Feel free to ping me either here or in chat after you read this comment so that I know that it is no longer needed and I can delete it.) — Martin Sleziak 11 secs ago

5:57 AM
@Martin: I think I didn't see that tag, it's definitely my mistake. — Asaf Karagila 1 min ago

3 hours later…
8:34 AM
@MartinSleziak I have added at least comment with a link to the archived version to both MO posts.

4 hours later…
12:57 PM
It seems that the tag-info for was taken from Wikipedia. However, the Wikipedia article has changed substantially since then - here is the current revision.
I suppose that in situation like this, Wikipedia still should be mentioned as the source, right?

8 hours later…
8:37 PM
@MartinSleziak : I don't remember creating a tag, but I probably also didn't pay attention to how much something was used if it showed up when I started typing; is it possible that something was using this tag and the tag was later removed? Anyway, I don't really want to get into tag management and advocacy. It would be nice to have a tag more specific than #ho.history-overview for biographies of mathematicians, but I'll leave that up to others to decide. — Toby Bartels 33 mins ago
I found no deleted question with tag: data.stackexchange.com/mathoverflow/query/858309/… And also searching for questions which had this tag in the past returns only one question: data.stackexchange.com/mathoverflow/query/851136/…
Also in January when I first noticed the tag, there was only one question: chat.stackexchange.com/transcript/10243/2018/1/30
I'd guess if we want to continue this discussion, it would be better to do so somewhere else - so that we do not leave here too many comments unrelated to the question (perhaps in chat). But probably the easiest solution is to leave the tag to its fate - as you suggested. — Martin Sleziak 17 secs ago
I'll add that on Mathematics there is (mathematicians) tag. I do not recall some similar tag here on MO - if we do not count (biography).

9:31 PM
@MartinSleziak @MartinSleziak :

@MartinSleziak : If you did the research, then you're probably correct. I'll use either tag if it exists, but I have no strong opinions.

Fair enough.
I contacted you mainly because it seemed likely that you consider tag useful - as the tag-creator (probably).
And as I mentioned, tags which only have single question are automatically removed after some time if they do not have tag-wiki. Some more details can be found here: meta.stackexchange.com/tags/tag-pruning/info
To be more specific, this post: Should We Zap The Low-Occurrence Tags? And only later it was modified in such way that tags which have tag-wiki are left even if there is only one question: Do not expire single-use tags that have a tag wiki.

@MartinSleziak : I understand, no problem.