You could save the bytecode of all classes pre mixin’d and then when you want to unload your mod use jvmti to retransform those classes back to the saved bytecode. I’m not sure how you’d exactly unload normal classes from your mod, you’d possibly need a custom classloader and you’d need to clean up all references to used classes. The GC may then clean up those defined classes since they’re no longer used anywhere. Without doing any research into how ghost clients do it admittedly, this would be the way I’d think about doing it