According to the Salesforce documentation, when a prospect is permanently deleted from Marketing Cloud Account Engagement, all the associated data and activities are also deleted and cannot be restored. However, if the prospect fills out a new Marketing Cloud Account Engagement form and submits their information, a new prospect record will be created with the same email address as the deleted one, but without any of the previous data and activities. The new prospect will have a new unique ID and will not be linked to the deleted one in any way. Reference: Salesforce documentation