[jiggawatts]

Apparently at C# implementation supports promise pipelining: https://github.com/c80k/capnproto-dotnetcore#features

It is missing many nice-to-have features however and is also apparently unmaintained, with the last release 16 months ago...

[naasking]

No activity doesn't necessarily mean unmaintained, it could just mean usable and stable/bug-free.