Here are some of the details:
Marketo doesn’t automatically delete people just because leads were deleted in Salesforce. Rather a field “SFDC Is Deleted” flag is set to true. You can trigger off this field to delete in Marketo if desired.
Delete Person flow action. This deletes a person in MKTO but you have a choice to delete in
Delete From SFDC flow action: This deletes a lead in SFDC but you have a choice to delete a person in Marketo as well.
If a lead is deleted in Salesforce (but a person is not deleted in Marketo) and then subsequently runs through the Sync with Salesforce flow action, then it would create a new lead in Salesforce.