Hashtags moderated at the relay level are nice. The dilemma with doing it at client level is the sheer number of possible hashtags that exist and God only knows how many relays each will connect with at a given moment. At the relay level, different topics or collections of topics can be moderated by different groups or individuals and presented/shared as discovery feeds. wss://aquaticchickentheory.nostr1.com is an example of 25+ hashtags, streamed in from public relays & filtered by my WoT.