There was a mixin for getArmPose of the net.minecraft.client.render.entity.PlayerEntityRenderer class, but the number of methods with the same name increased due to an update to Minecraft, so the mixin could no longer be performed. The IDE I use does not have a function to obtain the signature, so I do not know the correct signature, and even when I entered the proposed signature, it did not work.
Here's the code that didn't work either:
public class ArmPoseMixin {
@Inject(method = "getArmPose", at = @At("TAIL"), cancellable = true)
public class ArmPoseMixin {
@Inject(method = "getArmPose(Lnet/minecraft/client/network/AbstractClientPlayerEntity;Lnet/minecraft/util/Hand;)Lnet/minecraft/client/model/ModelPart$ArmPose;", at = @At("TAIL"), cancellable = true)