> However, it would be a significant bump in requirements for using the compiler on macOS: currently, the compiler can run on macOS 10.9 or newer. This would move that requirement to macOS 10.15.

Is this even relevant? For using a newer Swift with Xcode I'm forced to upgrade the OS anyway. Xcode absolutely refuses to be installed on previous OSes. Can you sensibly develop in Swift with anything else besides Xcode?

It's more a question of things like if you're building the Swift compiler in your CI system with macOS nodes less than 10.15, then you'll have to upgrade them. That can be problems if the build nodes you're building on are on hardware that doesn't support macOS 10.15. Now you have to buy a whole new Mac Mini or whatever just to continue building.