There’s no character limits on the protocol level, but there should be limitations to how much of that username is shown on the client side, most desktop clients and mobile clients are aware and have done that, others ignored it 🫡 https://nostr.build/av/f467b06eaa6df208a35b5f3d146d2f250b4652791c3f1119bd6db19e97cc7c50.mov