You can no longer link a terminated employee to a benefit, Hayley.
Please note that any outstanding benefit amounts must be reported directly to HMRC, as terminating an employee in QuickBooks doesn't automatically update their payrolled benefits.
Because of this, you'll have to manually complete the P11 form outside of QuickBooks to incorporate the benefits for terminated employees.
For detailed information about terminated employees with payrolled benefits, please check out the number 6. Terminate an employee with payrolled benefits section under this article: Add payrolling benefits in QuickBooks Online Advanced Payroll.
I've also included this article to guide you in generating payroll reports, for a detailed insight into your business finances: Run payroll reports.
Let us know if you have additional questions regarding the employee benefits.