hashing doesn’t solve the problem, it is not only about server storing phone numbers, but anyone being able to iterate over phone numbers, generate hash and ask the server for the contact, basically any contact discovery is at the same time an anti-feature, there are enough of such messengers out there, it is better if Delta Chat tries to cover the other need:
a safe messenger for family and kids where no one can discover your family members and scam them, do fishing or manipulate children, ex. ask for pictures etc.
also no public groups or channels so kids can’t end up discovering inappropriate content
so for the time being these features are not wanted in Delta Chat, this is not a social network but a private messenger