We have a protobuf type where we added a field, and then never implemented the features using it (so it was [deprecated=true] to hint that it should not be used). Several years later, the time has come, and now we do want to use that field after all.
Is it safe to just remove the [deprecated=true] and start using the field, or is that likely to break anything?
A field with the same type and semantics already exists on another message, so it would be very nice to use the name we gave it initially, rather than adding a new field and bloating the definition with two similar fields.
Edit: The proto3 language guide section on options has this to say:
deprecated(field option): If set totrue, indicates that the field is deprecated and should not be used by new code. In most languages this has no actual effect. In Java, this becomes a@Deprecatedannotation. In the future, other language-specific code generators may generate deprecation annotations on the field's accessors, which will in turn cause a warning to be emitted when compiling code which attempts to use the field. If the field is not used by anyone and you want to prevent new users from using it, consider replacing the field declaration with a reserved statement.
The only thing your clients will "notice", will be the missing deprecation warning in Java that they may have been used to, if they are still using the deprecated field. All fields are optional since proto3, so this should not break anything.