Thats a theory which I dont see much evidence for. Let me give you another theory about its effect. I would propose, that at least since the rise of the internet deplatforming leads to people getting pushed into far right echo chambers. I think its not just ineffective, its highly counterproductive. This unreflected feelgood nonsense is fueling the growth of the far right.
Since you brought it up, lets look at the topic of ethnic cleansing and how deplatforming would treat different people and what the result is. Say someone was never interested in politics or isnt that old and has some naivete left traveled to an area with extreme ethical or religious conflicts witnessing it first hand. An intimate view of decades old conflicts where parts of the civilian population are at each others throats and in some places even the threat of massacres is still very real if it werent for massive police or military presences. Take your pick from northern Ireland to some places in the Balkans to the variety of African conflicts with an unimaginable level of hate in some areas. Picture school children needing a police cordon on their way to school to escort them through screaming protesters because they have the wrong ethnicity or religious affiliation. Once people are personally affected or witness something they find atrocious they get motivated to think about it. How could the situation be improved? Talking with the people in the region he hears a specific mantra very often. As long as we still live door to door this conflict will continue. So the persons asks himself what could be possible solutions? The current situation is clearly intolerable to anyone with a sense of empathy. The person reads up on the conflict and its a decades or even century old issue. Quite alot was already tried, you can read books upon books of articles how the situation might be improved and about the numerous campaigns that were already completed. And still here we are today. So what were other regions that had the potential for ethnic conflicts but which are now resolved peacefully? A short look into the history books and you learn this was often achieved by deportations. You might not even have to look far, the formerly German provinces in Poland or Czechoslovakia dont have a any conflicts today, on the contrary. So apparently moving one of the groups is the solution. Sure this was often accompanied throughout history with atrocities, but back then horrible regimes and dictatorships were in power, now we have a properly functioning governments, those atrocities are a day of the past. Just horrible stories from the darkest days of humanity. We never had such a peaceful period in Europe and everyone knows we reached the end of history. So why not relocate one group and ensure permanent peace? So he asks, why dont we just deport every xyz in zyx?
My worry is, how many people on the left are still capable to explain to him why deportations and ethical cleansing are not just not a reasonable thing to do? Why his conclusion is wrong? Instead of just screaming Nazi and publicly shaming him? Could you? With deplatforming he is told that what he is talking about is called ethnic cleansings and he is a horrible Nazi for even mentioning such a thing. So he gets banned and has to look elsewhere for a solution to the problem he witnessed. He finds one of the isolated fringe boards. They are the only place to talk about it. While granted there are alot of Nazis, who cares, you find morons everywhere and they get banned on the platform as well, so he is obviously not in a Nazi board himself. He talks a while and finds some people who agree with him, who tell him that the mixing of inherently different groups is the core issue. He saw it himself after all. You just have to look as far as the Identitarian movement who put a lot of effort into discussion guidelines on how to convince people. Believe me if i tell you, they do know how to debate with someone, you cant cling to the cliche of the drunk skinhead.
That is of course a rather unbelievable story, who witnesses one of those conflicts after all? They are often shitty holiday destinations. But how many have had negative personal encounters with people who fit the role of a migrant or Muslim? The story is the same everywhere with every topic, we dont live in a perfect world and the far right is readily available with easy convenient answer for perceived or real problems. Are you still able to convince someone in a discussion about refugees and womens rights? And with convince I dont mean explaining someone why it is wrong to say something. What deplatforming is is peer pressure. You dont convince anyone with that. You just convince them that you have no answers yourself and to keep their mouth shut till an opportune time arrives. Before the internet that meant never being able to talk with anyone about that in your village or town because the neighbors might find out, with the exception of maybe a more extreme pub round. Your only real option was to look for a straight up Nazi Kameradschaft in the wider vicinity. That was a big step to take. Today they can easily look for more "reasonable" people or even join a major party. Deplatforming at its core leads to people getting targeted for what they say. The people get combated, the ideology behind it stays untouched. If we want any hope for the future that doesnt include a civil war or living in a fascist dictatorship we should look hard at switching that. Combating the ideology and convincing the people. Granted those debates are difficult and furthermore, a horrible past time. Most people dont want to talk about such atrocious things and dont want those discussion to happen in their living room. Just not having these discussions and excluding people who want to talk about it is much easier. Especially if you can feel good about yourself by going the easy way. The person vanishes from your view and becomes someone elses problem. Until they are all our problem.
You have a hypothesis, that deplatforming stops the spread of far right ideology. Thats a hypothesis we can easily test, we dont have to rely on your gut feeling how your policies affected the rest of the world around you. I think we can agree that we are just witnessing for the past few years an extreme rise of the far right across the globe. We are faced with openly far right parties which have made unbelievable rises in parliament and are in quite a few places on the way to becoming the strongest party and with that, will someday likely be the government. They already are the government in some places. Openly authoritarian politicians get elected and unthinkable thinks are happening like separating children of migrants from their parents and putting them in prison camps. And children dying due to lack of care in those facilities. I am sorry if i have to burst your bubble, but the current situation is a fucking emergency, the house is on fire and what we are currently doing is clearly not working. That leaves us with the question why deplatforming, exclusion and public shaming currently doesnt work? There are basically a few options as i see it (shamelessly stolen from a infamous German blog for people who are bored at work).
1) The strategy is valid and would work if it wasnt for those traitors in our midst who dont go along.
2) The strategy is valid and would work we just have to convince more people to join in.
3) The strategy is fundamentally broken and does not work.
4) The strategy is working, we just have to wait to see results.
If you see more options, please do share. I mean it. The situation is to damn severe for 4) we cant go on pretending like everything is fine and the situation being no different from combating the emergence of a Nazi youth club in small towns in the 80s or moderating a voluntary association in form of a board. If you have hopes for 1 or 2 i have to disappoint you. As an anti authoritarian myself let me tell you I sure as hell wont rally behind censorship. There is no authoritarian solution to the problems we face. While my view of the state of the world is granted horrible, I am sure i am not the only one who thinks this way. While the divide in the left between authoritarians and anti-authoritarians was not really a topic for the generation after the fall of the Soviet Union it is very real.