The community is actively working on "Eaglercraft 1.19" and "1.20" projects, which will pave the way for 1.21. For now, the best experience remains , which offers the best performance and the largest player base.
Some servers use plugins to allow 1.8 clients to join 1.21-compatible worlds. You won't see the new blocks, but you can interact with the community. eaglercraft 1.21
Eaglercraft 1.21: Is the Latest Minecraft Update Coming to Your Browser? The community is actively working on "Eaglercraft 1