It's always been cross-platform; GCC has supported it for decades. Apple's Cocoa libraries are Apple-centric, but there have long been alternatives like GNUStep.

Apple-centric? The libraries in iOS and macOS, for sure. However, the language itself can be used anywhere where GCC is available.

FWIW Objective-C predated Apple (or NeXT which adopted it and brought it to Apple) and was pared by Smalltalk.

A few years ago I ran into Steve Naroff in a diner and that’s one of the first things he said to me: ‘I’m still amazed we got Objective-C++ to work’ (for both political and technical reasons).

Was it ever widely uses? I was glad we did it and am glad to see that someone appreciated it!