Why does my iPhone 8 tell my Apple Watch that it’s out of date even though it’s on iOS 12.4

This may be due to corrupt data on the phone or the watch. Your watch may also be out of date in terms of hardware. Best way to know what is going on is contacting Apple support online, over the phone, and or in store. Genius bar reservations are best made through online support. Good luck hope this information helps.

Apple set it up so you know when it's Time to buy a new iPhone.