For good reasons I need to boot into Catalina or into High Sierra on a new iMac 2020, 27", T2. So far all attempts have failed. "Secure Boot" is set to "No Security" and "External Boot" is set to "Allowed". I have tried a couple of bootable High Sierra Installers, but Catalina just shows a "No Entry" sign after selecting the Installer drive and then turns off the iMac. I have downloaded the High Sierra Installer some time ago using my old MacBook Pro 2012. The Original High Sierra Installer is not anymore available in the App Store. I have also tried to install High Sierra using TM. It is a backup of my MacBook Pro 2012 with High Sierra (NO APFS). A message comes when I choose the Backup in Recovery mode, saying: this mac does not support starting from HFS. I do not think by converting my old MacBook Pro 2012 from HFS+ to APFS and then backing up with TM will solve the problem. Is it that the original High Sierra Installer must be downloaded from the same iMac where i want to use it and not from another machine? I am desperate. I have tried CCC, Superduper. NO Way. Unthinkable there is no solution to be able to boot into Catalina or into High Sierra on a new iMac 2020.
Your sticking point is actually much simpler than that.
No Mac can boot to an OS older than itself.
There's the occasional edge-case when a new Mac is released right on the cusp of a new OS, but otherwise, it's simply not possible. There are no drivers for the hardware in the older OS, as the hardware hadn't been invented yet, so Apple stops you from even trying.
Your only solution would be to run High Sierra in a Virtual Machine.
Sad but true, same answer as stated by @Tetsujin in other words, obviously this rule holds:
You can safely go back without risk to any version that came out after the Mac model came out. Else you run the risk that the old OS X doesn't know what to do with the new hardware features and becomes unstable.