That's true in principle. In practice, I've seen Emacs undergo non-trivial changes and yet none of my personal Elisp code has ever broken. I'm not sure how that

That's true in principle. In practice, I've seen Emacs undergo non-trivial changes and yet none of my personal Elisp code has ever broken. I'm not sure how that happens—perhaps the core abstractions are sufficiently simple that they don't need to change much themselves and only higher-level things change, so code written against the core abstractions doesn't get broken.