[tenebrisalietum]

Hosting your own with the limited scalability it entails is the only way to go.

For a true freedom-of-speech arena to exist anywhere, it requires a focused, detached, impassionate intellectuality from its participants, and an understanding/agreement that speech in and of itself is not equivalent to anything in the real world (speech may represent X but it is not X). This is a mindset and really a skill, and it is not one everyone has or even wants, and not one a lot of societies care to specifically nurture.

Individuals with this mindset can gather in places, but places really dedicated to the "craft" require heavy filtering, because this intellectuality is typically actively discouraged and often suppressed with great energy, so genuine "practitioners" are rare. Ruler-subject dynamics seem to be how large human groups tend to work and emotion is much more easier to guide people with than pure intellect/logic, hence the rarity.

You cannot trust yourself either: even in a non-totalitarian society where it's possible to get around that roadblock at great personal expense, it's quite likely that you may think you have the required mindset, but really would rather use the arena to vent things that you cannot in less free places; most likely because some consequence of your society's ruler-subject dynamic that you are in is A) being more visible than normal to you (subjects in a perfect society would not care that they are not the rulers, no society is perfect) and B) causing you discomfort.