This query is a bit tricky because it could refer to two different things: a search for the to the popular browser-based Minecraft clone, or a request for a technical guide on how to host/play it. However, since Eaglercraft is currently based on older versions of Minecraft (like 1.8.8 and 1.5.2), there isn't an official "1.21" version of Eaglercraft yet.
Was this the kind of you were looking for, or were you looking for a download link to a specific beta version of the project?
New mobs that add variety to combat. Crafter Blocks: The long-awaited automated crafting system. eaglercraft 1.21
Some users use cloud computing to stream a full 1.21 Java instance to their browser. The Verdict
Eaglercraft 1.21: Is the Latest Minecraft Update Coming to Your Browser? This query is a bit tricky because it
As of now, as a standalone, stable client. Beware of websites claiming to host it, as they are often just re-skinned 1.8 versions or, worse, phishing sites.
If an Eaglercraft 1.21 version were to release today, it would include some of the most transformative features in Minecraft history: New mobs that add variety to combat
Eaglercraft relies on legally sensitive decompiled code. Each new version requires a massive amount of manual labor to "port" the Java code into a web-compatible format.