if they call themselves a platform and deleted my game for illegal reasons under the "platform" umbrella, could i seriously sue them for doing so? the fact FB and google are on the firing line right now seems to indicate "yes"
You can technically sue anyone, so the question really is would the case hold up. With your example precedent, the answer is no. FB and Google are part of an antritrust lawsuit. Essentially, they are such large companies that the plaintiff is claiming that they represent communication on the internet, and that messages going through their platform should be regulated by the government.
itch.io is a small business that represents a fraction of the industry. It would be effectively impossible to make the case that by us limiting who can post on our platform prevents those people from posting their work at all. There are many other game platforms, and it's also relatively easy to set up your own website & payments as well.
Also, as a side note, you've referenced that we would be deleting a hypothetical game for illegal reasons. This is incorrect. We are free to choose who we host and do business with. We have every right to delete any account we want. All private business typically have this right. Running a business is about trust though, so we don't go around deleting accounts without reason. We establish trust with our users by having a clear terms of service and setting a standard through precedent. Sometimes it can be a little tricky, but in those instances we take those cases individually because we want to be fair with every party involved.