Wed Aug 02 2023 17:06 Sock breakthrough!:
The closest match I've found to the old Muji recycled-yarn socks are now Uniqlo's "melange socks"; a mix of cotton (80%), nylon (16%), polyester (3%) and spandex (1%). They don't feel as heavy as the previous champion (Muji right angle pile short socks), and I wore them through a recent heat wave with no problem. They even look like the old Muji socks, with a gradient of yarn colors, which makes me think they're manufactured with the same process.
Uniqlo melange socks are available as short socks and the misleadingly longer half socks. They are a little larger than the old Muji versions, which is okay with me as I always thought the Muji "short socks" were a little too small for my feet. Apart from that, the only real difference is the cotton-dominant fabric mix, where the recycled-yarn socks were mostly polyester.
Thanks to Sumana for dragging me into a nearby Uniqlo; otherwise I would not have found these.