We have no tag for content that asks about published 3rd party content and we don't need one
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 the information is needed, put it in the question body
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.