Naven
Answers
Is there a messaging app where strangers cannot contact you?
Yes — any platform with no public directory and no open direct messages. Naven is built this way: there is no list of members to browse and no way to reach somebody without their exact address or username.
Most platforms offer this as a setting rather than a property, which means it is off by default and can be turned back on. A platform that never had a directory cannot have one switched on by accident or by a product decision next year.
The cost is growth. A directory is how a service gets people to find their friends, and not having one means every connection has to come from outside the platform.
That is a real trade, and it is the one Naven has made.