Ancient Windows versions that users likely still use programs for since they have not updated their software in a few decades (or the company that makes them no longer exists, or the software works so why break it?).
Win16 API is an ECMA Standard
Sun Microsystems in its feat to hurt Microsoft's 32-bit OS eventual monopoly decided to standardize 16-bit Windows API as ECMA-234. Initially seen by Microsoft as Sun being mean and hostile, this would really predate the Microsoft take over the world Borg style by a bit (Microsoft would eventually try to EEE Java by creating an incompatible and broken Windows only Java-like language which eventually just turned into C#, win the browser wars and just halt web innovation standards wise, and many other evil things). This is something that will remain with them for quite awhile even as they dive into the seas of open source.
However, in light of today this move by Sun was actually a good thing because it completely documents the important bulk of Win16 programming. Since MSDN nuked all of the ancient API information years ago, this is really the only reliable place to find it in. Over time it has been noticed that programming for ancient systems is just lost. You usually have to find ancient books for the stuff, if they are even being sold as used in bookstores online and offline.
Windows 3.1 is cooperatively tasked, although I am not sure if 386 enhanced Windows has preemptive multitasking (it likely does since you can run DOS programs in it).