Untick "Restock Refunded Items" by Default

Leaves the "Restock refunded items" box unticked when you refund an order, for shops where returned goods are usually damaged or not resold.

Code

add_filter( 'woocommerce_restock_refunded_items', '__return_false' );

You can still tick the box for an individual refund when the items do go back on the shelf.

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It is added inactive: review the code and settings, then switch it on.