Requirements change. I’ve seen 10+ year old proto files at google still being used by services that are still being actively developed.
Over those ten years your data model has evolved. Your first client, the one whose data model you were imitating when you added the required field, was deprecated five years ago and turned down last year. You have a dozen or a thousand client services, each using you as a backend in a slightly different way. Are you sure every single one of them is going to require that field? Are you sure the field will even be semantically meaningful for their use case?
Over those ten years your data model has evolved. Your first client, the one whose data model you were imitating when you added the required field, was deprecated five years ago and turned down last year. You have a dozen or a thousand client services, each using you as a backend in a slightly different way. Are you sure every single one of them is going to require that field? Are you sure the field will even be semantically meaningful for their use case?