When a property is removed from a feed, by default, we will automatically untick the 'On Market' tickbox on a property record:
This means that the URL will still exist and be accessible, but the property won't appear in searches or shortcode output.
If you DO NOT want properties to be taken off of the market automatically you can tick this option in the import settings:
If you want properties to be drafted or deleted when coming off the market and not left in the system you can change this setting in the import settings:
This page also has a bit about how to redirect off market property URLs to the search page.