Jesse Butterfield

> Reading the field behavior docs, it sounds like it does not affect anything on the wire, so my guess/assumption is that besides just changing the types like `message: Message`,...

My understanding is that extensions are still supported by proto3 strictly for use in custom options. [google.api.field_behavior](https://github.com/googleapis/googleapis/blob/master/google/api/field_behavior.proto) is proto3 syntax and it's produced by google.