Full text search has been merged in #Mastodon main branch, and will be in the next (and final?) 4.2.0 beta 🎉 It is opt-in, so it will take some time to be filled with people content as they enable their profile to be indexed, but this was one of the most wanted Mastodon features for some time. We plan to deploy it to mastodon.social and mastodon.online in the coming days to have a bit more feedback on it and see how it behaves in the wild.
You could make the argument that they’re technically different things, but I think in practice they are the same. If you’re looking for something specific, then yeah search is gonna be more effective than hashtags. But if you’re just looking for something to read, you could search for “fediverse” or trawl the #fediverse hashtag. Not everybody chooses to add hashtags to their posts so full text search is gonna find more posts than just a hashtag search (assuming full text search also returns matches on hashtag).