The best (or at least, most successful) answer to this has come through federation. Platform owners are allowed to create and moderate their own groups with their own distinct rules and regulation. From there, each group decides which of the other groups it would like to interact with, as well as ones that it would like to ignore. The result is a system where everyone can live happily; 4chan trolls can federate with 4chan trolls, mainstream instances can block objectionable content, and people who want to use both can freely make accounts for either group. The result is a pretty good balance of freedom with highly-moderated content, even highly sensitive groups have had a pretty good experience using this tactic, since if all else fails the group can defederate from everyone else and choose only to interact with other group members, or selectively federate with other groups that share their same values.