As a very long time React user, React.createClass is certainly gone. So are some old versions of the context API. Although it never broke any of the guaranteed behavior, Fibers broke a lot of behaviors that were not guaranteed. Upgrading the major React version in any very large application is likely to cause at least some issue to shake out.

I think the more correct thing to say, at least in my opinion, is that React never breaks functionality without good reason. If you are following the React best practices and not relying on unintentional implementation details, using hooks/lifecycles/APIs correctly, your application will not break for a very long time.

If there was a sufficiently good innovation that was a pure win with no downsides, I believe the React devs would do their best to integrate it in a minimally disruptive way... But just like the adoption of ES classes, I do expect that eventually they will drop support for "the old way" when the time comes.