Final answer:
Medicare and Medicaid have always been liable for payment in a Medicare fee-for-service system.
Step-by-step explanation:
In a Medicare fee-for-service system, the payers who have always been liable for payment are Medicare and Medicaid. Medicare is a federal health insurance program that primarily covers individuals aged 65 and older, while Medicaid is a joint federal and state program that provides healthcare coverage to low-income individuals. Both Medicare and Medicaid have been responsible for paying for services provided to Medicare fee-for-service patients.