As a coder at strava fixing this would not be hard at all.

A global "Private mode" switch that sends zero data about anything at all while it is enabled. Your runs stay on device. All network calls are rejected. No data saved with it enabled will ever leave the device, full stop.

Every single app in the world should have this. It should be an OS setting that forces network calls to fail as well as part of the app review process that no data generated during a private session can ever leave the device.

They don't do that because they like your data for money.