Shadowbans have existed for literally 40+ years. There have been multiple forums, platforms, channels, BBSes which try to make an egalitarian system where moderation is clear and transparent. These systems all universally fail because it fails to function at scale or it becomes easily gamed by bad faith posters or it falls apart because you get the truly fixated and toxic posters that tear your community apart.
The problem is you assume every actor is rational. That they all want to simply post in good faith. The moment you make that assumption you become a fool, someone that will be manipulated and controlled. You will get people gaslighting you into thinking they're the heroes while they've been sending threats of violence to other members of your community.
There is no good solution. I doubt there will ever be a good solution because it's trying to solve an inherent problem in the human condition.
You're right the existence of shadow bans is old, but you're wrong that it's critical for a forum. Discourse doesn't use it, and Mastodon I believe is reviewable. So was Usenet. Kill files were controlled by users.
Also, Reveddit is used extensively on Reddit without subreddits falling apart. Mods of many groups will often link it to provide transparency into their work.
The problem is you assume every actor is rational. That they all want to simply post in good faith. The moment you make that assumption you become a fool, someone that will be manipulated and controlled. You will get people gaslighting you into thinking they're the heroes while they've been sending threats of violence to other members of your community.
There is no good solution. I doubt there will ever be a good solution because it's trying to solve an inherent problem in the human condition.