fwiw: I’ve just wondered why an example project didn’t have proportional digits with the “System” Font, even though the example has been run on macOS 10.14 and with Xojo 2018r4.
The reason ? If you set the Build-Architecture to “macOS, 32Bit” -> no proportional digits (but a monospaced “System” Font, such as before). Switch to 64Bit, and you’ll get the “System” Font with proportional digits.