The only question with left-4-dead-series tag was this question about Jesus room in No Mercy campaign, present in both Left 4 Dead and Left 4 Dead 2. Do we actually need it, as it no longer applies to any question and kinda overlaps with both left-4-dead and left-4-dead-2?
"Series" style tags are meant for questions that are relevant to many (or all) games in a particular series, rather than a single game.
This might not seem to make much sense for something like Left 4 Dead which (as of writing) only has two games, but look at series like:
- The Legend of Zelda -> legend-of-zelda-series
- Pokemon -> pokemon-series
- Portal -> portal-series
- Sonic The Hedgehog -> sonic-the-hedgehog-series
Each of these 'series' tags contain questions which span across multiple games, regardless of how many games in the series there are. The series tags are for series-level content - not individual games. So why shouldn't we have a Left 4 Dead series tag that does the same thing?
To apply the same logic as I discussed in Do we really need this “longplay” tag?
1. Does it provide a useful 'overarching' grouping of like-minded content?
Yes. As discussed above, series tags are for questions that span a series, not for questions about individual games. Likewise in reverse, individual game tags should be used questions about that specific game - not for questions about the overarching series.
2. Is the only (current) question a valid use of the tag?
I say Yes: The question is a terminology question that applies to the Left 4 Dead community and multiplayer regardless of whether Left 4 Dead 1 or 2 was being played. If/when a hypothetical 'Left 4 Dead 3' comes out, the question will just as likely apply to that game as well, assuming the glitch/mechanic remains in that game as well.
3. Do we keep tags with only one question?
We do! There is a relevant meta where Stack Exchange even disabled the script that deletes tags because it was causing us a lot of headaches.
4. Can [left-4-dead-series] be useful as a 'specific' topic area?
I believe so, for the points discussed above.
Thus I think the tag is relevant and useful and should continue to be used for it's one question.