Yes, they are useful
And No, they aren't useless 'because some games don't use them'. Subtags are decided on a case-by-case basis. Sometimes organically, as has been the case with Minecraft, and sometimes decided here, on meta, as with Diablo, TF2, Starcraft (and on the 'no' side: League of Legends and Overwatch).
I have the feeling these tags were created as experiments in Arqade's early days and kept around because they were already there.
Rather than relying on feelings alone, let's take a look at some of the earlier discussions.
A brief history of subtags
I think Jeff's post from way back in 2012 is one of the first that explicitly talks about subtags, and probably the reason we even have some subtags in the first place:
As a (very) rough guideline, I would say to vigilantly avoid extra tags at all beyond the game itself -- even in most case avoiding even two tags if possible -- until the game in question has on the order of 150+ tags at a minimum.
Once a game has hundreds of questions, it can possibly support some sub-tagging, but I would also urge you to use more game-specific words where possible...
Obviously our tagging structure has changed slightly since then - subtags are now prefixed with the game title or an abbreviation rather than being generic enough to apply to a wide variety of tags. This was followed up by a discussion specifically calling out the Diablo 3 subtags: Should we allow tags for a specific class in RPG games?
To which StrixVaria had this to say:
These tags add useful information. For example, I am very interested in Diablo 3, but not at all interested in things having to do with the Barbarian. If I mostly play as Witch Doctor and Monk, I can watch those tags specifically and add my expertise to those classes while not being bothered with questions about Wizards, who I don't know about.
This is very similar to races in StarCraft...
Here, we can see a few core uses of subtags:
- Easy filtering of questions you're interested in
- Easy filtering out of questions you're not interested in
- Getting Expert's eyes in front of questions they can answer.
A new Era was dawning
Around the same time, community members were working to sort out the tagging mess, and following this discussion there was a period of time known as the Great Arqade Tag Cleanup (of 2012, 2013 and 2013-2014) - removing meta tags and ambiguous tags that could apply to many games, like multiplayer, money, wizard and so on.
The majority outcome being that game names take priority, with very few exceptions used for sorting/filtering purposes (achievements, terminology etc).
The Status of Subtags
But what happened to subtags? Well, they stayed, because they adequately categorised their questions and did not fall into the trappings of non-game-specific topic areas. That doesn't mean they slipped under the radar: subtags were then brought up for Why do we have tags like diablo-3-demon-hunter?, for which Oak had to say:
The primary reason for tags is not for searching. You can just search for "diablo 3 demon hunter", no square brackets, and it will work just fine. Tags are primarily used for filtering - e.g. if someone is interested in all Demon Hunter questions she can "star" the tag (follow it), and then questions about it will be tinted differently and it will be included in the "my tags" tab of the unanswered questions page.
Likewise, if someone wants to filter away all questions about Witch Doctor, because he doesn't play that class, he can mark that tag is ignored.
So, why diablo-3-demon-hunter and not just demon-hunter? Because "Demon Hunter" is not unique enough to Diablo III. And even if it is, monk and wizard definitely aren't. And now this may collide with, say, the hypoethetical wow-wizard.
The case-by-case basis
The lack of similar tags for other games is telling, I think. If these tags were useful, we might've seen similar tags pop up for league-of-legends champions, terraria weapon classes, or overwatch heroes.
Again, I quote StrixVaria:
The difficulty here is where do we draw the line? In League of Legends, there are 90-something heroes each with unique skillsets and strategies. Clearly we shouldn't have separate tags for each hero. I think Diablo 3 is going to have enough quantity of questions and few enough class options to warrant distinct tags, but I don't think a blanket policy is going to be able to be created. We will have to analyze games on a case-by-case basis to come to a reasonable decision for that game.
We haven't consistently applied subtags to every game because not every game needs subtags. I dunno about Terraria, but let's look at a few games I can find, as well as the ones where subtags were discussed and ultimately decided against:
Note that I'm explicitly leaving off "sub" tags that are used standalone (e.g. grand-theft-auto-5 & grand-theft-auto-online, the recent Fortnite decision etc.
...these numbers don't look impressive enough to justify keeping the subtags.
How do we judge metrics on subtags? What makes a 'good' use of subtag vs a 'bad' use of a subtag, based on how many questions it's on?
Metrics get murky for subtags because it's hard to define thresholds. What if a tag with subtags gets 10 new questions and pushes the pre-defined meta-approved subtag ratio below that threshold? Do we then remove all the subtags? What if the next day it gets 50 questions about a new weapon or something that would've been tagged with a sub-tag?
As such I can only see two clear points where a subtag is utterly worthless and should be removed:
- When no questions use the sub-tag
- When ALL questions use the sub-tag (consider a synonym instead!)
The fact that a quarter (25%) of TF2 questions use sub-tags seems like a pretty fair use of subtags to me. I happen to know that they're applied to every question where they make sense - I went through and checked every TF2 question a couple of years back and regularly check the main tag for new questions and update where necessary.
I think we should continue to use sub-tags for games on a case-by-case basis, as we have done for years now. They provide useful filtering features for game experts & power users - grouping together questions they can answer as well as helping to filter out the ones they aren't interested in or can't answer. Seeing as our primary goal is to get expert answers to questions, this is the system working as intended.