Wait so, this tag you didn't like didn't even stop you from finding the fic? It didn't clog up your search at all because it wasn't even in your search when you found the fic you were looking for? What's the problem exactly? You're approaching this with a library lens but it's not a library! It was never even intended to be a library!
Additionally, it doesn't show up when you're searching for regeneration or limbs because it's a one-off tag and therefore isn't linked to the rest of the tag network. I suppose it would be a problem if you put it in the general search, but you'd also be catching anything with limb in the title, or limb in the author's name, too. I think this is coming from a place of multiple misunderstandings of how tags work from both a technical and a cultural standpoint.
And that's a problem, isn't it? I shouldn't have to be immersed in a culture to use the system. You've traded usability and user experience for ... a cultural in-joke. "Hi, this is AO3, and our tags aren't anything like anyone else's tags, but we're still gonna call them tags" is a problem. It's like if I made a search bar and it returned random results. It says search, but culturally, we give you random results. That's how we do it.
But it's not a problem because it doesn't affect your ability to search. One-off tags do not enter the tag search results. I'm super confused why this isn't obvious and intuitive to you..
Additionally, it doesn't show up when you're searching for regeneration or limbs because it's a one-off tag and therefore isn't linked to the rest of the tag network. I suppose it would be a problem if you put it in the general search, but you'd also be catching anything with limb in the title, or limb in the author's name, too. I think this is coming from a place of multiple misunderstandings of how tags work from both a technical and a cultural standpoint.