Stack Exchange has no freedom of speech. User probably tried to explain that you are an anonymous user using their public service, they can ban or delete anybody for anything (except not for being black or so-named "non-binary" and similars). But they can ban/delete anybody even without saying a reason. They can also suspend you for identifying yourself as "ultra-conservative". But if you would identify yourself as black, they could still suspend or delete you - if they do not say the reason, or if they do not say the true reason.
This is what the company can do on the U.S. law, but they only rarely interact with the communities so directly.
It happens more often that harassment happens on many, seemingly unrelated channels, but not directly from the company. These root obviously from the same source, but we have no way to say for sure, exactly where from.
The best what you can do: mods and the others can do, what the company allows to them, and it is unlikely that the company would interact with you so directly. The system is a big pyramide of power, and all the levels in this pyramide are bound by the rules given by the upper levels to them.
Understand these rules, and defend yourself, using them. For example, it was imho a complete mishap to attack a unix se mod for a chat suspension for which he said that not he did it. Imho, he actually helped you by giving the info that not they were the suspenders.
I could get the list of your account and I see where are you banned. Among these 4 sites, I would first check the meta.stackexchange.com mods, but note: it is only an educated estimate and I have no way to say anything for sure about it. Side note: meta.stackexchange.com mods can chat-ban you only on the chat.meta.stackexchange.com, but not on the chat.stackexchange.com, however there are MSE mods being mods also on other SE sites.
If you defend yourself, while you keep the rules, then the system will clearly help the perpetrators, but no so far that it would break the rules binding even them. So, if you get a harassing comment, then flag it as harassment. The mod can decline the flag, but can not delete the flag, and it leaves a trace, that he treated you unfairly. If you estimate revenge downs, flag it - the mod will decline it, but also this leaves a trace. If you are insulted, flag it as insult - no mod will leave a trace that he allowed insults on the site.
My experience is that if power users harass you, they stop it in some weeks. The power user harassment is typically bound to a single site. If you leave, and then later come back, they will start it again. Here the important thing is: there is a major cultural difference between us and them. For them, it is an ordinary way of the fight. For us... in a normal world, we never would do that, not only because it is disgusting, but also because it does not come in our mind. If someone says mostly evil, then we attack the most what he says, but not all.
The important thing is, that these power user circles are doing this "community harassment" because it is okay for them. This also means that it is actually not so evil for them, as for us. If you've fought a lot, you learn each others' mind and behaviors, not only you learn them, but also they learn you. And while you are in still unfriendly relation, on a deeper level they accept you as one of them. You won't be a member of the inner circle, but you will be someone for them who has a name. The sad thing is, that in some sense, you really become one of them. :-(
If mods seem hunting for you (for example, by suddenly honoring minor "heated arguments" by suspension), then my experience is that they stop it in some months. These hunts are typically cross-site ones. Maybe someone said to them in their chat room, that you are allocating too much mod/reviewer effort. Or attracted their attention that you are an "ultra-conservative". You can expect mod hunt if the previously tolerated nuances (like using the "braindamaged" word in a meta comment, or talking about the suspension of another user in the chat) get suddenly enforced with full power, and this happens simultanously on multiple sites.
Other type of the mod hunt if you get multiple suspensions without a reason, or with a crap reason, on the same site. These are typically following the usual 1 week, 1 month, 1 year structure. The mods probably want to defend themselves by doing everything "as usual". If you got the 1 week, followed closely by the 1 month, you can be sure: the goal is the 1yr, and you will likely get it for anything the mod can find. Be extremely careful for some months. These hunts are probably the own initiatives of a single mod.
The worst what you can do if you flee. With it, you feed them. If you got a revenge down for your meta post, then write yet another meta post. If you got a suspension for an "insult", then next time be twice so active, but without "insults". Another bad idea is to delete your account. Why? You get exactly the same if you simply don't use the site - but without giving up forever that you once come back and use your votes.
I nearly never got any useful answer on the /contact form, but also this leaves a trace. The possibility, that yes, you can contact the system any time if there is a mod power misuse, creates enough motivation for the mods to not go too far. Note also, even if the upper level (mod/CMs) decline your request, it is quite possible that they note it. You have no way to know, what else did they do, so the expected outcome is still more than nothing.
Once I got a serial revenge down on another SE site. I flagged it. The mod rejected the flag, saying that they can not do anything. A week later, I've seen a young meta site member, with whom we mutually disliked each other (although there were no major conflicts), being suspended for "voting irregularities". The mod was not allowed to explain to me, what he did, and I have no way to know, if he was suspended for serial downing me, but I think yes, it was the reason.