Thanks for doing that investigation @brson! I'd personally be fine for private-in-public or transmute-from-fn-item-types to both become hard errors as they've been on stable for at least one release (and they will be for at least one more b/c of beta).
The others though I agree that it may be a little soon to move them to a hard error as they're only sitting on beta. This does mean that they're guaranteed to still give warnings for a whole release, but it's probably good to let beta hit stable so anyone who only compiles on stable has a chance to catch up before we break nightly.
Although if the amount of code supporting these is blocking progress in other aspects, I could see making an exception and turning them to a hard error sooner. They will after all still have an entire cycle on stable as just warnings.