Question for anyone who knows about insurance costs on the employer’s side

I am a full time employee at 35 hours per week and have insurance through my company, where I’ve been employed for 23 years. I plan to maybe retire in 2-3 years. Recently my company went from 3 days in the office and 2 working from home to 4 days in office and 1 at home. I’m sure that soon they will require 5 days in office. I am considering asking my employer if I can go to part time, working around 15 hours a week (in the office) with my pay being cut accordingly. Obviously would love to stay on the health plan with the firm paying whatever they pay now for me, and me paying whatever I pay now too. I know 15 hours isn’t full time, but is it a law that they couldn’t keep paying my insurance? And, would it be a huge ask to ask them too? Is it a huge cost to them? It’s a big company with 3 offices in the U.S. and one in London and it’s Cigna. If they will laugh me out the door to ask them to reduce my hours but keep paying their portion of my health insurance I wouldn’t ask. But I don’t know if that’s a big deal or not.