The most definitive guidance I have found is in Internal Revenue Bulletin: 2004-2 dated January 12, 2004.
Health Savings Accounts
Q-27. Are health insurance premiums qualified medical expenses?
A-27. Generally, health insurance premiums are not qualified medical expenses except for the following……In addition for individuals OVER 65…….the EMPLOYEE share of premiums for employer sponsored health insurance, including premiums for employer-sponsored RETIREE health insurance CAN be paid from an HSA.
For reference I found an old Boglehead post: HSA’s-I Didn’t Know This!, dated March 11, 2017 by ThePrune and from the Health Equity HSA Guidebook, pages 71 and 112.
As a retiree over age 65, I believe I will be able to reimburse myself from my HSA for my share of the health insurance premiums from my employer-sponsored retiree health care (FEHB) plan.
Kaneohe, thanks for your comment. Any others?