You can't ascertain what's true without free speech.
Suppose you start with a true belief. This belief, like any other, has to propagate from person to person to sustain itself. The truth is messy. During each hop, people present the best, cleanest version of the truth in the belief. After enough hops, the belief stops being the best version of the truth and starts being a truth-flavored falsehood.
If you want to optimize for your beliefs being true, you can't just prohibit their opposite. Even if you don't care about truth per se and want only for your beliefs to spread fastest, you should want to make sure your beliefs stay true-ish, because if you decouple your beliefs from truth, then they become so mutated and useless that they prompt people to whom you propagate the idea to seek out other ideas on their own.
Yes, it's easier than ever for bad ideas to spread. What people who use words like "disinformation" miss is that this ease cuts both ways. What makes it easy for opposite beliefs to spread makes it easier for false mutations of their own beliefs to spread.
Even if you can ban opposite beliefs, you can't ban mutated forms of your own beliefs: attacking the mutants looks like attacking the truth forms, yet, because the mutations smooth over the messy parts of the truth, the mutant versions of your belief will out-compete the original within the bubble of your ban.
Truth alone doesn't stop the mutants because it operates on long time horizons and small gradients. Only strong contrast can distinguish truth from appealing falsehood, and only competition with opposite beliefs can establish it. Trying to establish this contrast within your belief's framework against a near-mutant looks like gatekeeping, pedantry, or disloyalty. The near-mutant is always too close to its parent to justify the social cost of an all-out fight.
Furthermore, once a mutant version of your belief has taken over, the original version itself looks like heresy. Deformation occurs in small steps because it happens unwittingly in response to intuitive incentives. Reformation occurs in large steps because it happens wittingly in response to observing incoherence. You can't ban near-mutants, but you can ban far-mutants, and reformers trying to jump from the endpoint of a long line of near-mutations back to the original form get the original form and themselves banned.
So now what? If you think it's hard to defend your true beliefs against opposite beliefs, think how much harder the job will be once you can't wield truth as defense. By default, beliefs win and lose as they become extractive and appealing in cycles. Truth allows an idea to win despite being costly. Without truth as a benchmark, why would anyone prefer your belief over upstarts that promise fewer rules and more fun?
If anything, people will prefer opposite beliefs to yours because you've been the one calling false beliefs true just because they descended from true beliefs and because you're the one telling them to shut up and get in line. Even if you start out with the true belief and the opposite beliefs are all false, you lose.