> Philosophically, checking that a field is required or not is data validation and doesn't have anything to do with serialization

But protocol buffers is not just a serialization format it is an interface definition language. And not being able to communicate that a field is required or not is very limiting. Sometimes things are required to process a message. If you need to add a new field but be able to process older versions of the message where the field wasn't required (or didn't exist) then you can just add it as optional.

I understand that in some situations you have very hard compatibility requirements and it makes sense to make everything optional and deal with it in application code, but adding a required attribute to fields doesn't stop you from doing that. You can still just make everything optional. You can even add a CI lint that prevents people from merging code with required fields. But making required fields illegal at the interface definition level just strikes me as killing a fly with a bazooka.