A PT may be superseded by another. For example, obfs4 superseded scramblesuit.
The maintainer of a PT may no longer be available. This was the case for FTE.
Eventually we should have a process to make this decision.
The retiring process
Inform the community by sending an email to tor-talk@. Also inform the relay operator community by sending an email to tor-relays@. This will give operators a chance to shut down retired transports.
Update Tor Browser. Also don't forget to update the list of default bridges.
