All Macs run macOS, previously known as Mac OS X, and have done so since the introduction of the first version of Mac OS X in 2001. Before that, Macs ran Mac OS 9 which came out in October 1999 ...
A modern version of OS X: openconnect is should work on most recent OS X versions. It has been tested and seems to function correctly as far back as 10.9. Mac OS X ...
The latest M4-based Mac computers can't run older versions ... signing into iCloud so that you can run older macOS versions with all your settings and data available via iCloud.