While answering another question here in meta, I noticed we actually have a absent-players tag. It had few questions tagged as it, though (and one tagged as missing-players, which should be a synonym imo, but I don't have score in any of them to suggest it). It draws attention that we have a good number of "What to do when a player can't come to a session" but most of them are not tagged with the absent-players tag. Is this because the tag is redundant or not useful at all or what?
Yes, the tag is useful.
First, it describes the problem very well. It's clear what we are looking at when we see this tag - we are looking at a problem about one or more players missing one or more sessions, and probably what to do due to this absence. This is as descriptive as, for example, powergaming, IMO.
Even contrary to powergaming, I don't think this tag is a specific subset of the [problem-players] tag. I went on and edited some questions adding the tag that I think are representative to my point here: the problems are closely (or exclusively) related to a player missing. How can I quickly adjust encounters for variable party sizes?, How do you deal with missing players?, How to deal with a player missing the first session of a campaign. Note that, as with How to deal with a player missing the first session of a campaign, for example, there isn't a problem-players problem, people just can't show up every session, it happens, and it's useful for us to know how to (better) handle it.
TL;DR: Yes, I think the tag is useful. I don't think we should be rallying to edit the tag into old questions, but if we come across one that is about players missing sessions, the consequences to it and how to handle it, we should add the tag to the question.