On my boat, the through-hull is there but disconnected. The head plumbs directly into the holding tank; no Y valve is needed.
The Y valve and vented loop are still there, too, but have the hoses disconnected. I have no desire to remove it; although there are things keeping my boat from being OD-legal, I don't want to add any more. By bypassing the valve, it makes yearly inspections in Chicago easier since I can claim the boat is incapable of overboard discharge.
