> I'm guessing you're thinking of various 100% proprietary channels, all of which suffer from the problem of being centralized and users and content can get arbitrarily banned or blocked for no reason.

You are looking for an open+federated protocol. Those definitely exist.

For example, an older one that I'm well aware has probably long-ago-peaked is XMPP. One can host their own XMPP domain and talk to other XMPP domains just like email can. It also has the ability to send content, presence, pub-sub notifications, offline messages (much like an email message) ... you name it. Storing contact lists and associations combined with the fact that messages come from authenticated sources by default means that spam is a lot harder to accomplish (or at least a lot easier to squash). All of this and XMPP is a 25 year-old protocol; This protocol can legally drink in the US. Just imagine what else exists today! :)

However, the problem is not what technically exists. The problem is what is popular. Email is still extremely popular and ingrained in so many places. It's easy to extend and put up a quick webpage that submits an email from some user-input. It's easy to send notifications/advertisements to. Attachment protocols have largely been sussed out. It's even used quite commonly as a second form of authentication. You can expect most people to have an email address while a much smaller percentage of people expect to have any other single-form of electronic communication.