Get the list of unused picklist options - picklist cleanup activity

Query -

SELECT thepicklistfield__c, count(Id) records FROM Account GROUP BY thepicklistfield__c

The values which you do not see under thepicklistfield__c are not selected. 

Use the below script to split -

Set<String> selectedValues = new Set<String>();

for (AggregateResult aggregate : [

    SELECT thepicklistfield__c FROM Account GROUP BY thepicklistfield__c

]) selectedValues.add((String)aggregate.get('thepicklistfield__c'));

List<String> notUsedValues = new List<String>();

for (PicklistEntry entry : Account.thepicklistfield__c.getDescribe().getPicklistValues())

    if (!selectedValues.contains(entry.getValue())

        notUsedValues.add(entry.getValue());

system.debug(JSON.serialize(notUsedValues));