>>> (This isn't just an academic problem - the problem of trying to evolve fields over time is the reason why Protobufs got rid of the idea of a "required" field.)

if this is the reason of the drop of the required field it doesn't seems smart to me.

They are conflating the definition of a scheme with the scheme evolution logic.

And there are multiple ways to deal with the latter (like adding a schema version, for instance)