A piece of the messaging to me from Strava is as follows:
If an existing user does not accept our updated Service Terms by May 25th, all API requests on behalf of that user will return a 403 Forbidden. If after that date, the user accepts the updated Service Terms then API requests on their behalf will work as before. We will be messaging users who do not accept the updated terms in the appropriate channels.
This means that if you do not accept Strava’s updated terms by May 25th, your account will appear to CityStrides as being de-authorized which will delete your account here along with all of its data.