Angry and disappointed that a push notification didn't advise that updated REMINDERS on iPhone would no longer work on my MAC!

Please be civil in respectful in your remarks. I work, and only visit these discussions when absolutely necessary, My last visit was years ago, a testimony to Apple design and execution. For one who rarely has to resolve computer issues, troubleshooting can be a steep learning curve. The purpose of these discussions is to diminish the steepness of that curve. Displays of behavior contrary to this purpose wastes everyone's time.

Have you been sleeping for the last ten days?

macOS 10.15 "Catalina" (on which forum you have just posted!!!!) has been out since October 7.

Reminders in Catalina work together with iOS 13. Just have to be logged in to the same iCloud account, of course.

Supported device: iOS 13 and iPadOS are compatible with these devices - Apple Support

FWIW, this warning screen comes up when you update to iOS 13…

and when you click "Learn More…"

Not angry. Just puzzled. All of my devices have been upgraded to most recent OS. Did not see any upgrade or reset notices on iCloud, but something could have been missed. Suggestions, please?

Sorry, puzzled, why? Your devices were all upgraded. Is there something that isn't working?

iPhone 6. Software update indicates all is current. iOS 12.4.2. Reminders unavailable. Is the phone too old to be brought up to current levels?

Yes. It's too old to support iOS 13.

edit: I should have said, no so much too old but the iPhone 6 is not supported.