A recent question asking about a homebrew/3rd-party class has been asked, and neither the 3rd-party or homebrew tags fit (based on their tag descriptions).
It feels weird to me not to have a tag, as it reads like something I just haven't heard of, rather than something separate from the base game. Do we have a solution for this?
I really don't see the problem that you are. We don't tag every question based on where in the rules it is found or what adventure it is found in (though we do have some adventure-specific tags I see these as exceptions and not something to try to emulate). Tags are for indicating the content of the question, not its source (with a few exceptions, system tags being one).
3rd party content is just content that WotC has not published. That, to me is not a useful distinction that needs to be made. As long as the author has clearly stated where the content came from in the question, then the question is answerable and no further tagging seems to be needed.
In fact, we had a tag for 3rd party content in the past and it was eliminated. Of course that doesn't have to be the final word on the issue, I just don't see the addition of a 3rd-party tag adding anything to our ability to link questions with expert answerers.
Do people consider themselves to be experts in 3rd party published content to the extent that they would browse that tag? Would people browsing the site find the tag useful for finding information about an issue they have? Does it improve searchability at all? To me, the answers to all those questions is "no" and thus I don't see a tag as value added here.
If a reader/editor thought there was sufficient cause for confusion about whether the content was published by WotC or not, they could always put the information about that in the question body itself.
For example, the question you indicate in your question has been edited to say "Protean Scribe (from Mythical Classes by LPJ Design)" which should be a strong and clear signal that this is a 3rd party source.
There's no tag for this.
We don't have a third party tag. As Rubiksmoose points out it was burninated — twice. Some notable publishers (like dreamscarred-press) might have dedicated tags about their material, but this is rare. This content is neither of these things so it has no particular tag.
You mention in comments you were surprised to see it mentioned because it was content you weren't aware of, but that's not necessarily a problem, and it's not something our tags will resolve if it is. I think editing the body to mention the publisher was a fine way to get the disclosure you're after.
Some things to consider:
This last point lead to a lot of debates when Critical Role's Blood Hunter came out apparently matching all three categories, despite them being theoretically mutually exclusive. Thankfully our tagging infrastructure kinda just sidesteps this epistemological debate though.
There are many cases (Pathfinder's Path of War and Spheres of Power are just two examples in my wheelhouse) where a 3rd party drastically changes the content of its source. The change is often so great that it changes who is qualified to answer the question. While some material might just be a new spell that neatly fits the mold, far more common are additional class/job/etc roles which can drastically affect game balance and, more importantly, the number of people who have actually interacted with the content to provide accurate answers.
I agree that the 3pp/3rd-party-product/homebrew tags don't need to exist, but individual publisher tags need to for the same reason we have game system tags. They change the answer base and let answerers know where to look for the content, for reference purposes.
