Teardown of new 21.5-inch iMac surprisingly reveals user-upgradable RAM, modular CPU - 9to5Mac
Amazon.com: OWC 32GB (2 x 16GB) PC19200 DDR4 2400MHz SO-DIMMs Memory Compatible with Mac Mini (Late 2018), 27" and 21.5" iMac (Mid 2017), and Compatible PCs (OWC2400DDR4S32P) : Everything Else
Amazon.com: Mid 2017 Apple iMac with 3.0GHz Quad Core i5 (21.5 inch Retina 4K, 16GB RAM 256GB SSD) (Renewed) : Electronics