It seems too specific
The help page about tagging says this:
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.
As a general rule, you should avoid creating new tags if possible, and new users are not allowed to create new tags. Even if you have sufficient reputation, you should only create new tags when you feel you can make a strong case that your question covers a new topic that nobody else has asked about before on this site.
The magic-initiate tag is too narrow. In particular, the Magic Initiate feat is not even the focus of that question (it's just one example of a feat), so it seems far too specific to be necessary in that case.
I suppose the Magic Initiate feat might be something one could be an expert on, but that particular question is not a good argument in favor of keeping it. The larger issue is that, as you point out, it'd be a system-specific tag for a specific feat; using the same logic, we could create individual tags for every feat/feature in every system that has more than a few questions about it, and that's clearly not the ideal goal.