Says who/what?

And the spec says plaintext usernames should be rendered as links?

Some day most people are going to understand that “I want to post something visible to everyone in the world EXCEPT these specific people” is not a viable or reasonable or even possible approach to communication, and any attempts to make it work are doomed to failure.

Most people who “self host” things are still doing it on a server somewhere outside their home. Could be a VPS, a cloud instance, colocated bare metal, …

I used Mattermost for a community project, but had trouble getting people to install/use/learn yet another client.

Web of trust solves this problem, until people start intentionally trusting AIs as much as they do other humans, at which point it’s no longer a problem.

“this day and age” is rapidly coming to a close

They could have used a mailing list or an rss feed or half a dozen other solutions that don’t require a special website or government app.