Yeah, the problem is that because "required" is checked at the protobuf parser layer, it will be enforced whether or not the application itself actually depends on that field being present. In practice this tends to escalate minor bugs into full outages. Like, there have actually been outages in Google Search, Gmail, and others that wouldn't have happened if all required fields were treated as optional instead.

I've written more about this here:

https://kentonv.github.io/capnproto/faq.html#how_do_i_make_a...