@iridescent_slime seems it's called Hemicorporectomy. Not sure if we want a more speaking name or if this is good enough, also due to the fact that people looking for this kind of thing might search on google for it to find the name first. Maybe this pic can be tagged with that plus triple_amputee. It's a specific enough concept that a single post would be justified for it imo, otherwise we'll never find it again.
I've added bisected for now, but I won't object if anybody wants the technical term here as well. I can't foresee it being a very common tag in the long run, though.
Tags like severed_head and severed_limb are for when the severed parts are present in the image. We don't use these tags for characters who are simply missing those parts.
For the sake of consistency, and because the tag doesn't really serve an essential function otherwise, I think it makes sense to treat severed_torso the same way and limit its use to images where the severed half of the character's body can be seen.