Is hibernation possible on a T2 iMac / Catalina?
I suspect that some power is still required to maintain hibernation, as distinct from 'off'. So you may be able to hibernate an iMac, but only while it's still plugged in.
The 'Resume' feature of macOS is designed so that desktops can relaunch apps and open documents and windows that were up at shutdown, 'resuming' where the user left off.
sleepimage does exist on my 2018 Mini, over 2GB in size: though the Creation and Modification date are both 8 November 2018, which was probably when I first turned it on.