Each transaction (insert/update/delete) on the Liquid Content creates a new version item along with a new slug value. All slug values are unique versions in the database. Thus, you can NOT re-use the same slug value once you create a new item.
This article provides information related to overwriting used Liquid Content slugs. Because once the slug values have been used for an existing content item, they cannot be used again.
If you have a previous backup of the Content Library where the previous slug values have not been used, you can import the backup file into the portal, and the slug values that were previously locked should be usable.
If you do not have a previous backup of the Content Library, submit a support ticket, and the Support team will forward your request to the SaaS Ops team.