In hashes the io dependency is optional, are we allowed to release hashes v1.0 with io pre-1.0?

@Kixunil said it yesterday menitoning an unstable feature (eg bitcoin-io-unstable). I cannot find the discussion and did not understand the gravity of the suggestion at the time.

This is a big deal because if we can then I/we can stop working on io and move on with stabalising the other crates.