Hi, I'm the author of Cap'n Proto, and co-founder of Sandstorm.io. When Sandstorm failed, I went to Cloudflare where I'm leading the Cloudflare Workers project. We're using Cap'n Proto (including RPC) pretty heavily there!

That said, it's true that Cap'n Proto relies on its users to contribute improvements to the implementation. We don't currently have a team who can dedicate their time to building out capnp implementations in every language.

At present, within Cloudflare, the heaviest use is C++, with some stuff in Go, Rust, Java, and Lua. Hence, if you look at the C++ implementation you'll see it's been pretty active. At the moment we don't have a JavaScript use case so haven't put effort into that implementation. But given that Workers itself is a JavaScript platform, it seems reasonably likely that we'll end up putting some work into the JavaScript/TypeScript implementation at some point down the road.