I started developing an iOS client for an API we use at work recently. Downloaded Xcode again, built a small POC Mac app and ran it. It wouldn't run because I am still on Mojave and the default settings are to build for 10.15. I was a little shocked/not surprised to get a "This application is not compatible with your software" type dialog from my own brand new code!
I really like a lot of the ideas that Apple implements, but the upgrade treadmill is notorious and not one of them.