Do you mean a NeXTSTEP version for non NeXT hardware because there was a Windows version?
Improv 2.0 was only available for windows x86.
Lighthouse design quantrix for nextstep is available on m68k, x86, hp-parisc and sun hardware.
Then there’s an offshoot of lighthouse design quantrix which is quantrix modeller written in Java and available for windows and Mac. It costs $2500/year.
I’m learning improv on m68k and quantrix(the manuals interchange freely) on m68k/x86.