I don't think it's an abuse, RFC9110 defines 414 as a response for "refusing to service the request because the target URI is longer than the server is willing to interpret". Since adding a query string involves only adding characters, this seems fine; there's no stipulation as far as I can tell that all pages a server hosts must adhere to the same length. I'd be curious if any well-known clients interpret it that way though, and make caching decisions based on it. As far as I know, they shouldn't.

Obviously it's against the spirit of the thing, but I don't think it's wrong per-se.