I just found this comment by chance: https://github.com/capnproto/capnproto/issues/205#issuecomment-148587965
Rusty Russell was considering using protobufs and cap'n'proto for his Lightning Network implementation in 2015.
Discussion
Interesting
Cap'n'Proto is extremely cool and needs more adoption
But it doesn't support fixed size byte arrays.
I considered using on a project, but felt it lacks some very important documentation, like what happens when a node tries to communicate directly with another one (instead of routing through many more) and fails? Will it keep trying in the future? Will it not? Is there a limit for retries? Is there a timeout? How can I configure the timeout? Can I disable this behavior? Etc.