For a while Mac OS 10 had support to run Mac OS 9—- via nicely integrated emulation. Then Apple removed it.
I’d just love it if the current OS supported all previous versions, via emulation.
In source code form you may even surpass Win32 as code written for X11/Xlib and Motif (to some extent since that wasn't 100% compatible between different Unices) will still compile with little to no modifications.
And of course anything you write to run in a web browser has good chances to work in the future (client side only). Well, assuming Google doesn't completely take over and decide that they know better than anyone else if it is a good idea to remove stuff or not.
Also while not exactly OSes, but several platforms exists that provide isolation from the underlying OS madness. E.g. Java/JVM is an example of a platform that almost never breaks stuff. Languages like Smalltalk and Common Lisp also tend to be very stable. Free Pascal and Lazarus developers also try hard to avoid breaking code (there is still breakage but it is very rare and in almost all cases is about bugs in the compiler - personally i had only a couple of cases where i had to fix code over the last 15 years and that took me only a few minutes).
I think it has always been this way. Software does rot if not maintained. If it's not through API changes, then it's security issues.