target all players that have an item in their offhand with custom_data
the item would be this
minecraft:amethyst_shard[minecraft:custom_data={ice_gem:1b}]
but how do I use it in a statement like this
execute at @s run effect give @e[nbt=!{Inventory:[{Slot:-106b,id:"minecraft:amethyst_shard"}]}] minecraft:slowness 1 3 true
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