Update the 'Last Modified By' field on a Lead record

There are certain actions on a record which may update the 'Last Modified By' date field on a record, and some which may not. 

These changes will update 'Last Modified By' (date) field on a record

• Edit/Save of the record manually or via API.
• Workflow or Process Field updates, Approval Process.
• Converting a Lead. Note: This value will be available in reports and exports but not on the converted Lead record.
• In lightning, when the owner read the record first time on UI because "IsUnreadbyOwner" flag set to False until the lead owner reads the record first time.  This does not occur in Classic

The changes mentioned below will NOT update 'Last Modified By' date field on a record

• Adding or Completing an Activity on a record.
• Adding a Lead to a Campaign.
• A formula field getting updated on the record due to an update on a field in a related object.
• Submitting a record to an approval process with no field update.