We’ve pulled with a 6’6" and a 6’3" bed for years now with NO slider or extended hitch, just a B&W turnover or Companion… Goosenecks (6’9" wide) with tapered noses, and fifth wheels (8’6" wide) with cut back corners. Never had an issue, and we have been at darn near 90* a handful of times over the years.
But most of the time, we DO NOT want to be “jack knifed” near, at or past 90*, and it is generally not hard to avoid. Being jack knifed is not easy on the trailer axles, especially on a triple axle trailer. Get out and look sometime when you are backing at sharp angle, and you will often find your trailer wheels torqued in opposite directions. Which is also why you should always “straighten out” after you have backed sharply, in order to realign the tires.