How disabling Mixpanel account will affect current apps in the App Store? Is it safe to stop paying for subscription? How Mixpanel reacts when its server is not available? Is it any risk to crash app?
it won't affect your app in a negative way. If you cancel your subscription, your account will be set back to the free plan and your app will continue sending data. But you should definitely remove the Mixpanel integration then if you don't want to use Mixpanel anymore.