I think this is where the bookmarks feature comes in. Someone correct me, but I think you can have public and private bookmarks. So you’re able to search for things later. And if they were bookmarked publicly, it can be a signal for others to see.
Discussion
Right. The nostr protocol is fairly liberal in what kinds of content one can post. One could use replaceable long-form notes to encode an encrypted list of bookmarked posts for example. There are probably dozens of ways to implement bookmarking and personal search for the specific only-you use case.
The nice thing about likes in particular is that it’s an established paradigm. Likes are low-hanging fruit, UI-implementation wise. Some clients won’t implement likes, but many do. So a user using likes for bookmarks/search can more easily try different clients compared to a user relying on a more exotic implementation.