Don’t need that old picklist value muddying your record data? Problem solved: There’s
now an easy way to remove a picklist value from all records in your org. This process works for
standard and custom picklists (restricted or unrestricted). This feature is available in both Lightning Experience and Salesforce Classic.
From the Fields page of the object that uses the picklist, click the picklist name.
In the Picklist Value Set section, click Del next to the value you want
Locate the Picklist Value to Delete
To remove the value completely from existing records, select Replace value on
records with blank value. This option also removes the value from the picklist field
that users see.
Replace with a Blank Value to Remove It from Records