When you run a bulk update from Shopify to Etsy, Shuttle leaves sold-out Etsy listings alone by default. So a listing that is sold out on Etsy is skipped, even when its Shopify product has changed. The Update sold-out products on Etsy setting lets you include those listings in bulk updates.
Why a bulk update skips sold-out Etsy listings
Shuttle skips sold-out Etsy listings during bulk updates unless Update sold-out products on Etsy is turned on. The setting is off by default. A skipped product shows this warning: "Product is sold-out, Shuttle is set to skip sold out products."
The skip only applies to bulk updates. When you update a single product, Shuttle updates its Etsy listing even if that listing is sold out.
Turn on updates for sold-out Etsy listings
Go to Settings > Products and open the Shopify → Etsy tab.
Find the Filter Products by State section.
Tick Update sold-out products on Etsy.
Click Save.
Once the setting is saved, your next bulk update from Shopify to Etsy includes sold-out Etsy listings.
What happens to a sold-out Etsy listing when Shuttle updates it
A sold-out Etsy listing becomes active again when the update gives it stock from Shopify. If any variation has stock in Shopify, the listing goes live on Etsy and buyers can order it.
A sold-out Etsy listing with no stock in Shopify stays sold out. Shuttle updates the listing but does not make it active, so buyers cannot order something you have none of.
What the Update sold-out products on Etsy setting does not change
The Update sold-out products on Etsy setting only affects bulk updates from Shopify to Etsy. It does not change single-product updates, which always go ahead, or real-time stock syncing.
To check why a product was skipped in a bulk update, see See what happened to each transfer in Transfer History.