Afaik specialisation (in full generality) would cause soundness issues, so it's not even just blocked by the trait solver, it's also blocked by figuring out a 'slimmed down' proposal that fixes those.

And that's not even getting into the problem that it's a fairly controversial feature, since people are worried about terrible, hard to track specialisation trees. (See, inheritance.)

I think there is a bigger issue with specialization, and it's that nobody seems to agree in what the semantics should be. The orphan rules are clearly artificially limiting, but there is no formal description of a new set of rules to replace them, only proposals.