There's one restriction that this would be subject to that SO_PEERCGROUPID isn't. The SO_PEERCGROUPID is exposed for any process whereas PIDFD_GET_INFO ioctls (that includes the PIDFD_INFO_EXIT) option is only available for processes within the receivers pid namespace hierarchy. But in any case, enabling pidfds for such reaped processes might still be useful since it would mean receivers could get exit information for pidfds within their pid namespace hierarchy.
