There is a new feature released in Train:
https://docs.openstack.org/api-ref/identity/v3/#what-s-new-in-version-3-13-train

https://github.com/openstack/keystone/blob/stable/train/keystone/application_credential/schema.py#L32..L66

@jtopjian I wonder how exactly new options should be introduced: within an extension or directly inside the "applicationcredentials" package?