There is no harm in having the tag, really
A question tagged gm-techniques will still show up in a search of that tag even if it is also tagged pathfinder. Ditto it'll show up as a highlight even if you don't highlight pathfinder.
Now, if you explicitly ignore-list pathfinder, those questions will fall under the ignore list. So, in fact, I do exactly this, and part of why I do that (not just with Pathfinder) is that there are games dissimilar enough to my interests that questions they generate tend not to be relevant to my play. This includes questions about play advice as well as rules.
Tags shouldn't be critical to making sense of and scoping a question
A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.
So, right there: tags are tools for categorizing questions. They are not the primary way of stating what your question is about (that is the title), nor are they the primary way to express your question and any supporting details (that's the question body).
Tags also aren't a special field for giving the question stats that you're not allowed to put elsewhere or anything. You can, in fact, say what system you are playing just in the question (and someone will come along and tag it later, to categorize it). It's okay to leave that out of the question body and lean on the tags, but I think we should do that as a matter of convenience rather than, like, a hard-and-fast policy that says certain things only ever go in tags.
The site has a decent search feature (it also shows up on Google very easily). The tags are just little bonus sticky labels to making browsing stuff more fun.