I would be very hesitant to call Cap'n Proto "abandoned". The Cap'n Proto developer is actively building a platform on top of it, and implements features in it as necessary, and as far as I've seen, actively works with pull requests for other features as well.

https://github.com/sandstorm-io/capnproto/commits/master https://github.com/sandstorm-io/capnproto/pulse/monthly