Why I care: Because changing the default UI input controls or changing their placement or changing how the software solves problems every few years is disruptive to efficient use of the software. I have to re-learn where those controls are or how to solve my problem in the new design.

Some examples I know that maintain good backward compatibility in their UI/UX: Vim, Emacs, Hacker News, lwn.net.

What other examples do you know?