In #303, the spec was updated to include functionality for in-seat trip-to-trip transfers via the addition of two new values 4 and 5 for transfer_type in transfers.txt.
This PR proposes two changes, one large and one small:
(Large) Drop transfer_type=5 from the spec. In the original PR discussion Add trip-to-trip transfers with in-seat option #303, there were two feed providers who utilized transfer_type=4 but there was never a producer who utilized transfer_type=5. Per the spec guiding principles, speculative features are discouraged. I acknowledge the potential use-case for transfer_type=5 but I'm not sure it's been fully proven out. I'd be willing to keep 5 as a reserved value if we think there is someone actively working on this.
(Small) Revert the change that made from_stop_id and to_stop_id conditionally required. After discussions with the original change authors (link), it became clear that the intention was that stop id fields are still required, but that they just shouldn't refer to a station for the in-seat transfer types.
See the related discussion from the #gtfs Slack channel and also in MobilityData/gtfs-validator#1266.