geckolib-neoforge-1.21.1-4.6.6 crashed neoforge 21.1.61 with no other mods installed
https://pastebin.com/raw/XzKcguYD

Does this issue still happen with latest NeoForge?

I can't seem to replicate this?

It's working fine for me in neoforge 21.1.71, using geckolib 4.6.6

Yes i'm getting the same issue on NeoForge 21.1.37:

Caused by: org.spongepowered.asm.mixin.injection.throwables.InjectionError: Critical injection failure: Callback method geckolib$wrapArmorPieceRender(Lnet/minecraft/client/renderer/entity/layers/HumanoidArmorLayer;Lcom/mojang/blaze3d/vertex/PoseStack;Lnet/minecraft/client/renderer/MultiBufferSource;Lnet/minecraft/world/entity/LivingEntity;Lnet/minecraft/world/entity/EquipmentSlot;ILnet/minecraft/client/model/HumanoidModel;FFFFFF)Z in geckolib.mixins.json:client.HumanoidArmorLayerMixin from mod geckolib failed injection check, (0/1) succeeded. Scanned 0 target(s). No refMap loaded.

I can't seem to replicate this?

It's working fine for me in neoforge 21.1.71, using geckolib 4.6.6

What does your gradle file look like?

Try neoforge 21.1.61 or older

I mean that defeats the purpose lol

If it only has errors on old versions, then don't use old versions

Especially since 4.6.6 was literally to fix a bug caused by a change in NeoForge