To my understanding, the reason why Danbooru doesn't have a "transgender" tag (well, it used to, but as an alias for "genderswap", before this was done away with) is most probably because their tags are strictly visually-based, i.e. tags refer to what is clearly seen in an image, and practically never concern themselves with the narrative or characters' background information.
Compare that to Dynasty Scans' tags, which cover both the visual and the narrative aspects.
EDIT: For the record, Danbooru seems to lump transsexuals (whether pre-op or post-op) under the "genderswap" tags, as evidenced by this post, which corresponds to the first page of this comic on DS (tagged "transgender").
last edited at Oct 22, 2019 11:23PM