https://golang.org/doc/go1compat
> Security. A security issue in the specification or implementation may come to light whose resolution requires breaking compatibility. We reserve the right to address such security issues.
In general I am very happy that the big emphasis on a stable APIs was taken up by the community, and that we have a lot of stable packages out now (even though they might not be 100% stable like the standard library). Since I also have to work with NodeJs where changing APIs and packages are much more common, I came to really appreciate that fact about the ecosystem.
But this sort of issue is exactly the sort of real-world review and hardening which justifies having a namespace for stuff to go _before_ it becomes stdlib.
/x/exp is expiramental.