There are no subtags. Any tag wiki that says or suggests a tag is, is wrong and should be edited to eliminate that suggestion. If that is not possible, it is a strong indication that there is probably something wrong somewhere. One of the primary tests for a tag’s quality is that it something that can be used as the sole tag on a question. See The Death of Meta Tags, where dependence is literally the first listed signifier of a problematic code tag.
In this case, it was warlock that was wrong; it never should have claimed that it was specific to “the” D&D class (more on the scare quotes here in a bit). There are a number of class tags with this problem, some of which have been cleaned up as they got noticed/as they were desired for use in another system.
On our site, because of the way system tags work, a number of tags probably can’t stand alone without a system tag—because you can’t ask about warlocks without knowing what a “warlock” is and that’s going to be system-dependent. This is a problem, but as far as I can tell, an unavoidable one. But while tags relating to such details are necessarily going to have to be paired with some system tag, it should never be precisely one system tag that it gets paired with—that is, it should not be a “subtag” of any one system.
Therefore, use of an alleged “subtag” is not an excuse for guessing a system tag—since there are no subtags to begin with. There are no excuses for guessing a system tag. You simply should never, ever do it.
Also, since it came up in the discussion, please read the tag wiki for dungeons-and-dragons. The question that was asked was not answerable as a dungeons-and-dragonswarlock question. Honestly, considering the changes in the descriptions of what is referred to by the term “warlock” across the editions of D&D, it’s hard to imagine that combination of tags ever being viable for a question, aside from a question literally asking for a history of that term’s usage in D&D and how the warlocks from each edition differ from one another. Hence my use of scare quotes earlier when referring to “‘the’ D&D class,” since there is no such thing as the one and only D&D warlock class.