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Replaceable was meant to hint at it being a shadow more than its type, so I think I agree with this. If custom block types facilitate special labels then it could still work for shadow blocks (if that ever actually makes sense).

Separately, should this be protected? I assumed we never designed Block functionality for inheritance since Blockly doesn't support custom BlockSvg class implementations. I'm wondering if this needs to be something overridable in the block definition instead.