The legendary creature tags should be used on pictures that feature said legendary creatures, not anything else. Random posts of Takanashi Kiara shouldn't have the phoenix tag just because she's inspired by it, much less things like Phoenix (Azur Lane), whose namesake ship is technically named after the city in Arizona and not the mythological creature. post #2887137 and post #196641 are good uses of it, but a lot of the tag is canon tagged nonsense (like post #5728339, seriously?)
These tags are always going to have gardening work to do because canon taggers can't help themselves. Qilin (mythology) has a qualifier and still gets things like post #5916842 and post #5853915 because Ganyu is canonically half-Qilin.
As for phoenix girl, two of the posts don't belong and the other two can probably be safely moved to bird girl. Otherwise the tag can be safely nuked/deprecated, unless anyone else thinks there's a genuine use for the tag with other untagged posts.