While the 1.21 client doesn't exist, you can use Eaglercraft (most commonly the 1.8.8 version) to join many exciting public Minecraft servers. Many servers support Eaglercraft connections, often through a dedicated WebSocket proxy.
Eaglercraft runs using open-source code and does not distribute official Minecraft assets directly, aiming to operate within legal bounds, though it operates in a gray area. eaglercraft 121 10 full