Question: is Objective-C post 2.0 being actively maintained outside of Apple?
Can I use the new features that Apple added in cross platform projects or open source environment like this?
I loved Objective-C very much (yes, really) and I'd like to continue using it after Apple phases it out
Yes as part of the GNUStep project [1]. You can compile Objective-C with Clang for any platform, including Windows, and link with the GNUStep Obj-C runtime. The big problem is outside of GNUStep and macOS you won't have any frameworks - not even NSString.
> I loved Objective-C very much (yes, really) and I'd like to continue using it after Apple phases it out
You're not alone. Unlike C++, Obj-C was a reasonable OO extension to C. It's a shame the language never received more love.