My issue generally boils down to it being optional and something instance admins and mods would have to opt in and maintain, because if it were a technical solution, it wouldn't be able to exclude instances someone has an issue with. It would also negatively impact the content variety due to different instance rules AND community approach.
E.g. there are several gaming related communities, but they all differ. Some allow bots, others doesn't. Some allow memes, others' doesn't. Some limit posts, others doesn't. And all of them are on different instances that have different rules and moderators.