I miss rectangular sleeping bags made with UL materials. They were the OG “quilts” when opened up, were more comfortable than a mummy when zipped as a bag, could be paired for couples, or layered for flexible temperature ranges.
UL rectangular bags solved all of the problems that the discussed “unconventional” sleep systems attempt to address, and more. Nothing sold today is as versatile. (Yes, a rectangular bag is heavier, but not much when built with a light shell.)
All of the modern rectangular bags I see are either summer weight and small (AegisMax E, 50F comfort) or heavy for car-camping (Coleman, et al).
The closest thing I have found is the large XL700 comforter by Cumulus. It lacks a zipper and is not one of their customizable offerings. Otherwise, it is a near miss.