Are extension declarations supported?

[timostamm]

Extension declarations as described on protobuf.dev seem like a useful feature to me.

But descriptor.proto still reads:

  // For external users: DO NOT USE. We are in the process of open sourcing
  // extension declaration and executing internal cleanups before it can be
  // used externally.
  repeated Declaration declaration = 2 [retention = RETENTION_SOURCE];

Is it possible that the comments are outdated? The documentation on protobuf.dev doesn't appear to give a similar warning.

[honglooker]

ext decls should work in oss. @mkruskal-google can you confirm?