In iOS 13 Apple finally revamped Reminders.app, unfortunately only on iOS so far. If you ignore the warnings, update to iOS 13 and start Reminders.app it’ll tell you to “migrate” your Reminders data to the new version.

This comes with the nice side effect of breaking Reminders on all devices not on iOS 13 or not on the—currently unreleased—version of the new macOS.