Funnily this came up recently for us, our local postie was off for about a week (holiday) and his 1/2 round wasn't covered (he collects for other 1/2 of his job) those that can cover his round all either in isolation or covering full rounds, they mostly don't cover half rounds unless they can do two and have the spare staff, so we had no post for a week.
The absorption (I think it's called ) when delivery office staff go out and share missing rounds has gone as the isolation rate/booked holidays meant our D.O. struggle for enough staff currently. When our postie returned we got the backlog (2 items), most of our post is junk leaflets and the occasional odd thing that isn't easily done online, all important stuff is online.