Any new values entered into an unrestricted picklist field will cause new inactive picklist values to be created.
The only documentation I could find:
When inserting an unrestricted picklist field that does not have a PicklistEntry, the system creates an “inactive” picklist value. This value can be promoted to an “active” picklist value by adding the picklist value in the Salesforce user interface.
When creating new, inactive picklists, the API checks to see if there is a match. This check is case-insensitive.
When inserting an unrestricted picklist field that does not have a PicklistEntry, the system creates an “inactive” picklist value. This value can be promoted to an “active” picklist value by adding the picklist value in the Salesforce user interface.
When creating new, inactive picklists, the API checks to see if there is a match. This check is case-insensitive.
https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/field_types.htm (go to Picklist Field Type section)
https://help.salesforce.com/articleView?id=000340136&type=1&mode=1
