Author here. I'm now the lead engineer on Cloudflare Workers, and we use Cap'n Proto pretty heavily. The Workers team actively maintains and improves the Cap'n Proto C++ implementation.
With that said, outside of C++, the story is admittedly not as great. Go and Rust have decent implementations but are less-actively maintained at the moment. Other implementations are in various states of usability but many of them were projects that random people did for fun and then moved on from...
So, unfortunately, I am not sure I'd recommend Cap'n Proto for a project that has components in languages other that C++/Go/Rust right now, unless you're ready to actively participate in maintaining (and maybe building) the implementation for your language of choice.
Was researching flatbuffers a lot recently. Do you have any updates on thoughts about it.
Capnproto has a just a blurb about it. It looks more widely supported, crazy fast, and quite flexible. I played around with it recently, needs more development but the small amount of additional disk usage is a fair trade off to even faster (basically non existent) encode/decode times.
