Health data consent
When you first open Why Not, the welcome screen asks for your consent to use your health and fitness data to build and adjust your plan. If you set up Why Not before it asked, it asks once when you open the app. Apple Health then asks separately for each kind of data.
You can withdraw it at any time: open Settings and tap Health data consent, then Withdraw consent. Why Not stops reading Apple Health, so runs from other apps and their heart rate no longer come in. Everything else in Why Not works. Tap Agree on the same screen to give it again.
Your runs already on this phone stay until you delete your account. The copy your account keeps, your plan and your run stats, is deleted along with it. What Why Not wrote to Apple Health stays there until you remove it in the Health app: tap your picture, then Apps, Why Not, and Delete All Data from Why Not.
Profile, the gear, Settings, Health data consent, Withdraw consent.