There isn't any date for retirement as such - you can still order a trailer with a 5 or 7 pin round plug if you want to.any dates for when various plugs got retired though?
Round plugs haven't been retired, they have just become less popular over the years. Look at it the same way as Dencorub, almost no one uses it anymore because it makes you smell like old people, but Chemist Warehouse still stocks it.
Round plugs and sockets are the same - no one uses them because it makes you look like a Grey Nomad who still lives in the 1970s, but Supercheap Auto still stock them.
In fact, my Sierra (2018) came with a large 7 pin round socket and a 12 pin flat socket.