Witness data is more prunable than OP_RETURN. People have been sold a lie. The witness data is not included in the main txid, so it can be thrown away COMPLETELY while preserving verifiability of the supply. It is what would be pruned by a theoretical “non-archival node” You cannot delete an OP_RETURN without breaking the TXID hash and by extension breaking verifiability. So you must always store the data even if you prune it. Not that there is any incentive to use it, anyway, since it costs 4x more per byte.