I said it several times, but with Ivory's latest announcement, let me repeat it: we (the Mastodon team) are working on implementing Quote Posts. This is a much more complex feature than showing a preview for a link to a post, which is done at the moment by multiple clients.

It is a complex task and we have been working on defining the feature and the protocol-level details for some time. We are moving forward, and there are fewer hard questions to answers, but progress is there.