I was referring to using GNUstep as a framework/toolkit for Linux, not macOS and the reason i asked that was exactly because macOS has issues with backwards com

I was referring to using GNUstep as a framework/toolkit for Linux, not macOS and the reason i asked that was exactly because macOS has issues with backwards compatibility and GNUstep has the goal of copying Cocoa - my question was, "to what extent?" Linux is backwards compatible going back decades and glibc also largely maintains backwards compatibility as long as you are not using hidden APIs - and realistically if you stick with the common C stuff you should be fine in most systems (AFAIK outside of Linux, most Unix systems treat the C library as their "stable" API).