The story of "Eaglercraft 1.21.4" is less about a single download and more about a community's race to bring the latest Minecraft features to a web browser. While official stable versions like 1.5.2 and 1.8.8 are well-documented, 1.21.4 represents the cutting edge of community-made ports and "feature ports." The Quest for 1.21.4
You can host a 1.21.4 server using the official Eaglercraft (a separate download, usually a .jar file). eaglercraft 1.21 4 download
: The primary versions maintained by the original creator, lax1dude, are 1.5.2 and 1.8.8 . The story of "Eaglercraft 1