I've filed a bug here: [SR-11912] update_checkout doesn't run on macOS Catalina, or README is incorrect. · Issue #54330 · apple/swift · GitHub
This has worked for me before (though I haven't tried it in some time) and I just tried this on a fresh install of Catalina (and it failed). I think this may have something to do with update_checkout
becoming a python module. Has anyone else run into this issue?