In order to comply with some regulation (such as GDPR), team admins are now able to modify Data Retention settings.
You can find those settings in Settings / Teams
By default, retention is turned off. This is the normal settings and won't do anything to your account.
If you turn it on, then every prospect who has been inactive for more days than specified will get deleted automatically on a daily basis.
You can check each prospect to see how long before the prospect gets deleted based on your settings.
You can also decide if you want to have the ability to re-add the prospect or not. By deleting permanently the prospect, you won't be able to re-add it later. This is useful if you have auto-import spreadsheet still active or some automation re-adding the same prospects.