Homebrew is not an appropriate tag
From the tag wiki:
Homebrew, so called by analogy to home-brewing beer, is the creation of new game material by fans (not the publisher or licensed third party publishers) for an existing game system.
The purpose of homebrew seems to be to indicate that the poster is looking to modify the game in some way and requires some assistance from us to do so. It is NOT intended to be a flag that simply points out that some material in the question is 3rd party or unofficial.
The distinction in the tag wiki makes it clear that it the tag is only for content that is not published. Regardless of how you want to define published, we must keep in mind the whole reason we tag something in the first place.
Using homebrew as a "3rd party content" tag goes against the purpose of tagging
A tag is a word or phrase that describes the topic of the question. Tags are a means of connecting experts with questions they will be able to answer by sorting questions into specific, well-defined categories.
Tags can also be used to help you identify questions that are interesting or relevant to you.
According to the help center on tags, tags are supposed to deal with the content of the question only to connect question askers to question answerers.
Tags are not intended to be signposts warning people about certain aspects of the question that aren't directly related to the question itself. For example, having a tag saying "this is third party content" doesn't actually help to characterize the question or to help potential answerers find and answer it. That kind of distinction needs to be in the question itself not in a tag.
In fact, we used to have a 3pp tag, but it was removed for some of the reasons I have just discussed. See here for more: Should we get rid of [3pp]?
To that end, I wonder even about the usefulness of the dms-guild in this post. According to it's tag wiki it should be used only for questions about the DM's Guild not material from the DM's Guild. Using it in this way makes it a meta tag which is not acceptable use according to our guidelines.