I think you only made a case for having two or more levels of instance block, that already exist. One due to objectionable/illegal material that cannot be overridden, and another for something like threads where a significant number of users may not want to be opted in automatically, or want to block it due to purely ideological, non-illegal reasons, which would effectively be put in place by automatically adding the instance block to user accounts that can be removed at any time, which arguably can already be done with minor changes. That’s essentially what dansup is doing, complete with including a command for Pixelfed instance admins to apply the optional block to all user accounts.