Ironically, Protobuf does have field options for "required" and "optional", but over a decade ago (I guess shortly after it was first introduced), Google added a huge "do not use required! validate required fields in your own logic!" warning to the docs, because a missing "required" field throws out the whole packet. (And when it's time to deprecate a required field -- it's way more effort)

We originally used "required" for fields that must be present and it screwed us over later due to that. Switched everything to "optional" with the code checking for those fields' existence instead.