is
Offhand cannot be used as a SWAP target in any container on 1.15.2 and below.
https://github.com/ViaVersion/ViaFabricPlus/blame/b03638ee999f658856e8284f135bcbf55fc596a8/src/main/java/com/viaversion/viafabricplus/injection/mixin/features/interaction/container_clicking/MixinMultiPlayerGameMode.java#L101