Kind of a bad example. Firstly because you are comparing windows with the Linux kernel. The Linux kernel has excellent backwards compatibility. Every feature introduced will be kept if removing it could break a userland application.

Linus is very adamant about "not breaking userspace"

The main problem with backwards compatibility (imho) is glibc. You could always ship your software with all dynamic lobs that you need, but glibc does make it hard because it likes to move awkward and break things.

Glibc is one of the few userspace libraries with backwards compatibility in the form of symbol versioning. Any program compiled for glibc 2.1 (1999!) and later, using the publically exposed parts of the ABI, will run on modern glibc.

The trouble is usually with other dynamically linked libraries not being available anymore on modern distributions.