I vaguely remember that Yellow Box for Windows still existed with some WebObjects releases. Existed in the sense that you actually got ProjectBuilder and InterfaceBuilder and all the libraries and were able to build Objective-C applications on Windows. I’m not sure anymore though if this was officially supported in any way.
Here are some screenshots of it, including InterfaceBuilder http://www.shawcomputing.net/resources/apple/os_pictures/yb2...
According to https://www.betaarchive.com/forum/viewtopic.php?t=29049 there's an even older version branded “OPENSTEP Enterprise 4.2 for Windows” instead of Yellow Box.
edit: Neat, I never realized TextEdit was a Java Yellow Box app. Cool way to both test/dogfood and market the Java bindings: http://www.shawcomputing.net/resources/apple/os_pictures/yb2...
Also, it's not-even-mildly interesting but the Start Menu shortcuts are oppositely-spaced compared to the branding within the apps themselves for some reason — i.e. “Project Builder” vs “ProjectBuilder” and "YellowBox” vs “Yellow Box”. And it comes with a Bourne Shell! http://www.shawcomputing.net/resources/apple/os_pictures/yb2...
IIRC OPENSTEP Enterprise for Windows was a version before Rhapsody and was actually an official release. I had this on CD-ROM, I might actually still have it somewhere.
There was also a OPENSTEP Solaris for Sparc, but I think this was a bit more obscure, don’t have any firsthand experience with that.
https://www.flickr.com/photos/96579068@N05/13061732584/
I was working for a bank around that time and we used a trading system which was initially running on NeXT boxes, but we had it also running on NT 4.0 later on.
Edit: I think I even found some screenshots of that trading application:
https://th.bing.com/th/id/R.8d154655f6713bc112d178164e4ffe51...
This seems to be running on Windows XP, so that was quite a bit later. But if you look at the toolbar icons, the heritage is quite clear ;)
