Online in a forum format? I haven’t seen it exist, the medium simply isn’t conducive to it. It requires very heavy handed moderation, which in turn stifles political discussion. It’s why HN largely bans it all-together as simply not worth it.
See https://help.nytimes.com/hc/en-us/articles/115014792387-Comm...
Generally, when unmoderated, the loudest user(s) will drown out everyone else
Reddit, up until some years ago, was great for political discussion. Simply ignore low quality opinions and focus on the few people who have actual knowledge.
Then the "safe space" movement started and anything that wasn't mainstream opinion was banned all over the place.
Suddenly, political discussion became impossible, because you could not discuss any non trivial things without few of being misunderstood and your account getting banned.