Eric Kinnear (linked post; Apple) is the author of the HTTP/2 fallback for WebTransport, so it's safe to say that WebTransport will be available in WebKit at some point.
Great news! It looks like Google GRPC team is probably waiting for Safari Webtransport support to implement grpc over the web in WT. Even though Chrome has had it for 2 years they have kept saying they’ll wait until it becomes GA.