1.21.6 Quickfix for Spigot
I'm not entirely sure why Paper isn't working — I assume they've moved and renamed some classes. I attempted to create a working solution using mappings.dev for 1.21.6, but I wasn't successful.
Additionally, the first error thrown when loading the solution for v1_21_R4 - after renaming the obfuscated methods - is as follows:
Caused by: java.lang.ClassNotFoundException: net.minecraft.world.entity.player.EntityHuman at net.wesjd.anvilgui.version.Wrapper1_21_R5.
(Wrapper1_21_R5.java:**) at net.wesjd.anvilgui.version.VersionMatcher.match(VersionMatcher.java:56) at net.wesjd.anvilgui.AnvilGUI. (AnvilGUI.java:43)
-> Basically the Method Wrapper1_21_R5_Spigot#toNMS fails because the returned class's superclass EntityHuman has been moved.