OWC has prepared RAM upgrades for the 27-inch 2019 iMac already - NotebookCheck.net News
OWC 1.5TB (12 x 128GB) PC23400 2933MHz DDR4 LRDIMM for Mac Pro 2019 models
The 27-inch iMac 2019's RAM may be upgraded to 128GB - NotebookCheck.net News
OWC Announces RAM Upgrades for New 27-Inch 5K iMacs - MacRumors
Apple iMac Retina 5K 2019 and 2020 Memory RAM - Macfixit Australia
Amazon.com: OWC 256GB (2 x 128GB) PC23400 DDR4 ECC-R 2933MHz LRDIMMs Memory Compatible with Mac Pro 2019 and up Models (OWC29D4LR0256) : Everything Else
128GB Memory Upgrade for 2019-2020 Apple iMac (27-Inch)
Memory Upgrades for 2019-2020 iMac 27-Inch - Up to 128GB