It adds a massive suite of modules, including advanced AutoCity, specialized movement hacks, and niche render features that aren't found in the base client. The 1.21.1 Compatibility Situation
Minecraft 1.21.1 introduced several backend changes to the way the game handles data and networking. Because Meteor Client itself had to undergo significant updates to support these versions, addons like Rejects often experience a "lag time" before they are fully stable. meteor rejects addon 1211
Check the tab on the GitHub page to find the most recent "Build" that matches the 1.21.1 release. It adds a massive suite of modules, including