Despite the prevalence of its subject matter, I had somehow never noticed this tag before it attained a princely 808 posts. (As usual, this isn't as mysterious as it seems - checking the logs confirms that one user nearly doubled its size in the last fortnight, although it existed before then.)
It does arguably have a blacklisting use, since the ostensible purpose of fan_character (before it got polluted beyond all meaning) was to enable people to filter out the torrent of self-inserts when looking for porny fanart. This tag specifically indicates that the fan and canon characters are interacting sexually, instead of simply existing near each other, although I'm not entirely convinced that the sort of people who hate fan characters would be happy to see either scenario.
But it definitely needs to be aliased to something, since *_x_* tags are totally nonstandard on e621 and both canon and oc have been invalidated and aliased respectively. Do people think this still needs to exist, or should it just be merged into fan_character as soon as that gets cleaned up?