Note that this still (of course) requires following said person on said instance. So people won't be spammed by views they disagree with. Mastodon is no different than, say, Facebook there. You still need to explicitly follow people. Mastodon doesn't even have an "algorithm".
But I can't see any positives from being able to defederate like this. What is the main benefit to remove the freedom of your users to follow people?