From what I remember, character and relationship tags don't have to be "common" to be canonicalised (that's more a requirement for additional tags), as long as all the characters involved appear in the source text.

So a rare tag in a popular fandom can get canonicalised quickly, while a tag in a more obscure fandom like The Yellow Poppy can sit and wait until the characters have been confirmed by a tag wrangler who's either familiar with the source text or interested enough to do the research.

I'm a bit more surprised that a Phantom tag would have languished, but I suppose if there's a neglected corner of the Phandom, Love Never Dies would be it.