Why? I feel like the strategy with libs has been working quite well.
The list of issues that @nrc pointed to are largely ones that I recently created to exhaustively track every feature flag, just as we do with libs. Many of the flags are obscure, others are tied to larger constellations of features that should probably be stabilized together (e.g., there are several feature flags involved in SIMD support). I don’t see any particular urgency to take action on most of these issues.
For the example of unsizing coercions, I suspect the basic problem is twofold: lack of advertisement of the feature, and the fact that it’s a somewhat niche feature in the first place.
So far, the lang team has yet to go through a full nomination process for the recently-added tracking features, like we do each cycle on the libs team. I think the simplest step to take here is, next cycle, to do a full round of nomination and start tagging some of these issues for FCP in one direction or another. As we’ve found with library APIs, the FCP is a great way of focusing attention and spurring discussion, and – contrary to the presupposition of this thread – many unstable APIs are indeed seeing wide use already.