I'm referring to the whole UX of it, really. The experience for mobile users, the "join community" process for new users, the experience for the community owner, not just moderation but how easy it is to start a new community. You can go and start a new Discord community in a minute. It's free and with no restrictions on the number of members.
Anyone wanting to launch new forum software needs to compete with that.
And on top of that, "just self-host" isn't enough in an era where developers don't even want to self-host their own projects anymore, let alone some third-party forum. That's another thing that has -deeply- changed in the last 20 years.
Genuinely curious, what would that be?
From user perspective is there anything else to the forum than Read/Post/Search?
Or are you referring to administration and moderation tasks?