Hello there, charcharmusic,
Backdating a leaver is unavailable. Since the employee could have been already provided P45 in which they;re marked as left the business with HMRC.
You'll want to add the leaver as a new employee, with a new employee ID and new profile altogether.
Furthermore, you can also export the payroll data and re-import it in QuickBooks, then add the payroll you need to.
Check out this article for reference: Process a leaver in QuickBooks Online Standard Payroll.
I'll be here if you have more questions.
