hyper-h2 has supported trailers for a long time: v2.6.2 supported them just as well as v3.0.1 does. This error appears to come from mitmproxy itself, as you can see here.
I’m not a mitmproxy core so I can’t be certain, but if I had to guess I’d say that the likely explanation is that mitmproxy doesn’t support HTTP/1.1 trailers either. I certainly can’t find any reference to them in the codebase at this time.
Certainly there’s a question to be asked about whether mitmproxy should treat this as a warning situation, rather than an error. Perhaps mitmproxy could even just add silent support and have a TODO about exposing trailers in the dissector. Until that time, though, mitmproxy is not well suited for intercepting gRPC traffic, sadly.