Even Linus’s comments in that bug hedge things a bit:

> and that if people depend on interfaces we exported having side effects that weren't intentional, we try to fix things so that they still work unless there is a major reason not to.

(Emphasis mine.)

In the Flash case, they changed the direction memcpy works in, in a way that didn’t actually improve any performance (Linus did benchmarks to show it) and Linus’s point was that there’s no upside to the change, and only downsides, so on the whole it makes no sense to keep the changed memcpy.

You could imagine plenty of scenarios where existing behavior truly is broken, and show that no popular software depends on the brokenness (which is why distros are in a great place to do these kinds of smoke tests, they have thousands of packages they can test), and make a judgement call.

A “zero regressions no matter what” policy is impossible in practice due to Hyrum’s law, at some point you have to use common sense and draw the line somewhere sensible.