Manage Deleted Fields in Lightning Experience
Where: This change applies to Lightning Experience in Professional, Enterprise, Performance, and Unlimited editions.
How: Deleted custom fields initially are soft deleted and their data is stored until they’re permanently deleted, or after 15 days elapse.
Click Delete on a field name, then click Deleted Fields at the top of the Custom Fields & Relationships page to see a list of soft deleted fields. You can then choose to undelete or permanently delete the fields. If you choose to delete a field permanently, you are prompted to confirm.