The comment policy is provided in the activity using an ad hoc field 'commentPolicy'; since nobody else has stepped up to the plate with anything better. It's documented in our FEDERATION.md . You can try to inter-operate or propose something better. You can also  ignore it or even remain in the dark that comment permissions exist in the fediverse. We don''t care. We're not accepting your comments if they violate our local policy or our members' permissions.