By default, we look at each product variant and check if the option "Continue selling when out of stock" is enabled on Shopify. If it is, yes, this will be active for that specific variant. This is one of the ways this option can be enabled for a variant.
The other option is enabling this through your Rich Returns dashboard. Find your way from your Dashboard -> Return settings -> Variant Exchanges -> Force overselling. Checking the button will disregard settings in Shopify and always show variants as available for exchange.