I need to be able to extract past/terminated employees from Xero.
The documentation (https://developer.xero.com/documentation/api/payrollau/employees) states that "By default using GET Employees will return a summarised list of active employees." There are no Optional Parameters documented that allow access to past/terminated employees, however these can be seen on the web page Payroll->Employees->Past.
How are past employees accessible through the API?
A bit of a scroll down the page you've linked shows just how you can accomplish that:
Example of retrieving terminated employees
Using a where filter you can specify a status of TERMINATED. e.g.
Status=="TERMINATED"
This translates to the following URL once percent encoded:
https://api.xero.com/payroll.xro/1.0/Employees?where=Status%3D%3D%22TERMINATED%22
{ "Employees": [ { "EmployeeID": "30b19d1e-afc6-4df9-bb17-b8ab247730fa", "FirstName": "Adam", "LastName": "Sumo", "Status": "TERMINATED", "DateOfBirth": "/Date(-786931200000+0000)/", "StartDate": "/Date(1527897600000+0000)/", "TerminationDate": "/Date(1572825600000+0000)/", "TerminationReason": "R", "PayrollCalendarID": "d4a9437d-3b27-476c-ac15-db6439fac720", "UpdatedDateUTC": "/Date(1572911939000+0000)/" } ] }