Sure! You could certainly imagine extending Protobuf or Cap'n Proto with a way to specify validation that only happens when you explicitly request it. You'd then have separate functions to parse vs. to validate a message, and then you can perform strict validation at the endpoints but skip it in middleware.
This is a perfectly valid feature idea which many people have entertained an even implemented successfully. But I tend to think it's not worth trying to do have this in the schema language because in order to support every kind of validation you might want, you end up needing a complete programming language. Plus different components might have different requirements and therefore need different validation (e.g. middleware vs. endpoints). In the end I think it is better to write any validation functions in your actual programming language. But I can certainly see where people might disagree.