An implementation policy that is overtly not a stable specification changed.

Using “test" at some level of the domain name, including the top level if desired, has been the documented correct and safe answer for a long time, and .dev has always been an unnecessarily risky hack. (Amusingly, everyone else doing that hack got screwed because Google was using it as well and realized that if it didn't take control of the TLD and manage it to support it's pre-existing internal use, it ran the risk of someone else doing that.)