In case you were wondering how long you could keep on using macOS 10.14 “Mojave” as a developer targeting any Apple OS, the answer is “not too much longer.” I was presented with the dialog box above when trying to run the beta for Xcode 11.4 on my MacBook running Mojave.
I was doing all this as part of updating The iOS Apprentice, 8th Edition, a great book for people who want to get started building iOS apps. It’s available in both electronic and dead-tree formats, and when you buy an edition, you get updates of that edition for free!