This is why free speech is so hard to maintain. There are so many cases where you want to ban or censor something obviously repugnant, but once the power is created to do so, it will be abused, probably as soon as someone you don't like is now in charge.
"Permitted" is a strong word for activity enabled by a protocol. It implies some moral acceptance from the ones who designed the tool. I think a better word would be "possible". Similar to how e-mail makes it possible to send all kinds of content.
This doesn't really work because it doesn't allow a moderator to set the tenor of a community. People can filter out individual messages, but a dedicated set of bad actors can turn a community into swiss cheese or undermine discussion just by spamming, baiting and trolling and taking advantage of the variance in the level of tolerance for the bad behavior.
Communities function when there is a standard to which the community members adhere and when bad behavior is uniformly moderated away. Making each individual have their own moderation bubble is a recipe for incoherence, even with the improvements you suggest. Its also a lot of work.
You're describing the exact thing people do when logging onto a Mastodon instance. They're choosing to accept the moderation/federation policies of that instance.