I think Mastodon's low-discovery mindset (no public search, no QRTs) is overall good, but one thing I think I should be able to do is search posts that I personally have seen. Like if something appears in my Home feed, I've already "discovered" it. There's not particularly interactions one would want to avoid that are exposed by letting me search for posts I saw yesterday and lost the link to, except interactions those people were already exposed to.