I also tag things like that. But I don't mind spoilers and can enjoy 'mystery' stories even when I know beforehand what the secret character or pairing or whatever is.

In the case of 'mystery fandom' stories where the author obscures the fandom, I wonder who the audience is. Are there people who are interested in reading stories where they don't know the fandom and are willing to risk that it may be a fandom that they know nothing about?