Surprise! The iMac Pro has Upgradable RAM and SSD - The Mac Observer
Teardown of new 21.5-inch iMac surprisingly reveals user-upgradable RAM, modular CPU - 9to5Mac
Memory Upgrades for 2017 iMac 27-Inch
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
Memory Upgrades for 2017 iMac 27-Inch
YouTuber upgrades base-model 5K iMac with seventh-gen Intel Core i7 processor, 64 GB of RAM, and more [Video] - 9to5Mac