The 2017 iMac Retina 5K is listed with its known gaps
The 2017 27-inch iMac Retina 5K, which has no T2 chip, is on the Mac support page after a change merged into quattro. The display does not run at 5120×2880, sound is not functioning, brightness has no effect, and sleep hangs until a power cycle.
ahmadtv's change lists the model A1419. It has no T2 chip, so the installer sets it up like the other Intel Macs.
The display does not run at its native 5120×2880, because the panel is two tiles and the stock driver lights only one. The page names a community patcher, omarchy-imac18-3, for the display, sound, and brightness.