Showing in-stock and backorder products only
Excluding out of stock, but including instock and onbackorder products
Open up the Frontend search settings -> Custom fields panel
Click to create a New filter
For the filter name you can choose anything, we typed "Stock"
For the field name choose _stock_status
For the field type choose Hidden
For the field value enter:
instock::onbackorder
For the field operator choose EXACTLY LIKE
Save the field and the search settings
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