I think the number of discussions that are off-limits should be very small and platforms should be much much more transparent.

However, if allowing certain discussions means also allowing other discussions, I'm not broken up if sites like reddit were to ban a subreddit like conspiracy or at least try to reshape it to something much more objective.

If the owners a property decide certain views are abhorrent, that's their right, I can't think of a valid moral or legal complaint against that - it is their property. If we lack competition that is an issue of market competitiveness more than propaganda.