Strict mode was invented as a way of introducing a handful of "breaking" changes without them actually being breaking, since it's opt-in. JavaScript had some egregious design issues from the beginning and strict mode is a remedy to some of them.

But when most people say "breaking changes" they mean things that suddenly cause legacy code to stop working, and non-strict mode will almost certainly be supported ad infinitum, so I don't really count that. It's also almost certainly never going to happen again.