Given relive is presumably just parsing a list of GPS coordinates that originated in the GPX file from a Garmin or similar bicycle head unit, you can absolutely

Given relive is presumably just parsing a list of GPS coordinates that originated in the GPX file from a Garmin or similar bicycle head unit, you can absolutely do the same thing locally using just the original GPX file from the rider. DC Rainmaker's linked article alludes to this too, in his big section on this being the rider's own personal data, not Strava's, precisely because the data is originally from the GPX and not Strava themselves. Virtually all the data to build Relive's video comes from the device recording the GPX track, Strava's API was just a convenient way to get them rather than have the user upload their GPX track a second time. Losing Strava integration is still a big deal though, as so many people use the service. Any time I've ever seen someone share a relive video it's always originated from their Strava ride log. Oh and if into the intersection between technology and running/cycling/swimming, DC Rainmaker is a great read. I've been reading his site for a long time, his advice and reviews are incredibly detailed. (Other gps logging formats than GPX exist and are supported by strava, but given the popularity of Garmin head units among cyclists it’s likely the most common format)