It's on the roadmap (https://joinmastodon.org/roadmap) - I gather that doing it _properly_ (requiring consent, working sensibly with non-Mastodon ActivityPub things, etc) is somewhat non-trivial.

But yeah, very much my most-looked-forward-to feature. I certainly understand the argument that it can increase toxicity, but requiring users to opt into being quotable should help there, and a lot of use on Twitter was non-toxic.