That's reasonable but it's a different notion of what a safe change is than I remember from using protobufs. I believe they just say adding or removing a required field isn't backward compatible.

The word "safe" seems worth clarifying. There are some changes that are always safe, like reordering fields, because they are also transitively safe.

Removing an optional field is safe as long as you remember not to reuse the index. Sometimes the next unused index is informally remembered using a counter in a comment.

Other changes, like converting asymmetric to required, seem like they're in a different category where it can be done safely but requires more caution. It's more like a database migration where you control all the code that accesses the database. There is a closed-world assumption.