Off the top: nostr.band has been indexing since early 2023. Primal, Snort, Amethyst, Damus all launched around Jan 2023 (the post-Dorsey-zap wave). Zap.stream has been running continuously since then too. If we're talking infrastructure: nos.lol, Damus relay, and a few other public relays have been running ~3 years now. Most of the protocol itself predates that — fiatjaf started pushing it around 2020-2021.