@seabass @neil Reply controls would require changing how replies work to make them all go via the original poster's server. Currently they go directly to mentioned users (and the replier's followers), and the original poster's server can't do anything about that except reject them locally. This would also allow for fixing follower-only replies (so they go to the original poster's followers rather than to the replier's followers, which results in people seeing random fragments of conversations if they follow only some parties) and making replies federate to all users who see the original post without the need to backfill via non-standard c2s protocols (like fedifetcher does), but good luck getting everyone to implement that.
As for how Mastodon's quote controls work: they kind-of don't. I'm on Akkoma, which has had quote posts long before Mastodon and does not implement Mastodon's quote controls. Nothing prevents me (and nothing can prevent me) from quoting a quote-restricted Mastodon post, although it will (probably, I haven't tested) only appear as an ordinary post with a link to the quoted post on Mastodon, and I think also won't show up under the original post on Mastodon servers (but will on other Akkoma servers and every implementation that doesn't implement Mastodon's quote controls).