Manage your inactive picklist values and enforce limits on inactive values for custom picklists to improve system performance and overall health. You can now bulk delete inactive unused picklist values. Previously, custom picklists with many inactive values were difficult to maintain because you had to delete inactive values one at a time. This feature is available only for custom picklists with predefined values. This feature is now generally available.
Where: This change applies to Lightning Experience and Salesforce Classic in all editions.
How: From the Inactive Values section of the picklist field, click Delete Unused Values. After you delete unused values, if a picklist is unbound and the total number of inactive values falls below the limit, the picklist is set to bound.
Clean Up Inactive Picklist Values (Generally Available) (salesforce.com)