My two cents since I'm one of the users who adds UTAU to every Kasane_Teto. In fact, I specifically have a saved search to garden such vocal synth posts.
I actually tend to search copyright tags by default rather than character tags so it bothers me when any instance of a character doesn't have the "base" tag that I search with.
Removing the UTAU tag doesn't feel like the right thing to do here since there are Teto posts that aren't specifically related to the UTAU engine; i.e. original designs, events, holidays, etc (I would add song designs as well, but I guess that's debatable), but because UTAU is that base tag, these posts are also tagged UTAU. The logic here would be "tag Teto with UTAU by default, except in case of SynthV" which feels inconsistent.
I think normally this wouldn't be too big of a problem because upgraded designs usually represent a minority of posts among vocal synths, and even then they're usually somewhat close to the base design. But specifically in the case of Kasane Teto, her upgraded design Kasane Teto (SV) is directly responsible for her current surge of popularity and makes up a majority of her posts now. There are 9.4k of Teto posts, 8k of which are after SV was released 2 years ago compared to the 17 years of Teto's existence, and 3k of which are tagged with Kasane Teto (SV).
An alternative solution could be to create a Kasane Teto (UTAU) tag. I would've considered this unnecessary padding (and quite frankly rubs me the wrong way since the UTAU design should be the default without needing it's own tag, but perhaps that's just fear of change) but considering the stats above and just looking at the most recent 200 posts, 90 of those are SV compared to 51 UTAU. If the point is to search specifically for the UTAU design, this would be the most efficient solution.