There's a simple echo server demo at https://webrtc.internaut.com/wt/

If you want an example of an application that actively benefits from using WebTransport, there's a proposed video streaming protocol called WARP: https://datatracker.ietf.org/doc/draft-lcurley-warp/

The lack of good examples mostly stems from the fact that there isn't really that many publicly available server libraries yet. The two I can name off the top of my head are aioquic (Python) and the Google QUIC implementation used in Chromium and ENvoy -- the latter is sadly not trivially embeddable by third-party code (I work on it, and I've been trying to make it easier to use, but given that it's a large C++ codebase with a lot of dependencies, this has been taking a while).

I think it's too new. HTTP/3 has only recently been standardized, and Web Transport is built atop that. New tech, not many users yet. I suspect we'll see many web games utilize this in the near future.

It was only added to Chrome in January of this year and isn't in Firefox or Safari. So I suspect there isn't much beyond showcases for now.