I tested a few modpacks that use Spectrum and noticed that you mixin into it to apply your changes to the Bedrock Anvil, too. Neat!
With Spectrum 1.7.9 I made some changes to the Bedrock Anvil GUI to closer match the 1.20 vanilla code.

But because they were substantial changes, that broke your mixin. Sorry!

The changes: DaFuqs/Spectrum@c0dc6cd

Ah, thanks for the info!
Gonna have a loot at it right now.

P.S.
I love Spectrum, easily one of my all-time favorite mods!

Aha!
I was confused for a bit since the relevant canTakeOutput and addExperienceLevels code didn't actually seem to have changed and rebuilding my mod without changes magically fixed everything.

The things is: Going from HandledScreen to ForgingScreen means the canTakeOutput method now actually overrides the vanilla method, so it'll be remapped to method_24923, that's the breaking change.

I guess I'll take some time to rewrite some stuff using MixinExtras while I'm at it (those mixins are old).

Perfect, thank you!

Aight, made it backwards compatible too! Thanks again!