We've found that Genesys Cloud has a max limit of 1000 Contact List records which can exist for an Org, so are having to regularly manually remove old/historic Contact Lists to keep within the limit.
The DELETE api/v2/outbound/contactlists/{contactListId] would probably work, however is reliant on identifying the required contactListId's to submit into it, whereas ideally we're looking for the ability to remove Contact Lists within a date range/older than X date.
What you could do is call GET /api/v2/outbound/contactlists with a query parameter string like "?sortBy=dateCreated&sortOrder=ascending" or "?sortBy=dateModified&sortOrder=ascending" -- using an ascending sort order puts the oldest contact lists at the top of the results. Then filter for the contact lists that meet your date criteria and call the DELETE resource I mentioned above, passing their IDs in the id query param