I would like to install macOS 10.13 High Sierra on a Mac Pro 2013 that currently has macOS 10.15 Catalina installed. I want to do this because most of my application S/W is known to be compatible with High Sierra. Is this possible? Are there any special procedures or precautions I should be aware of? Thanks, Tony
Instead of erasing the Mac, you might want to get a spare external SSD, format it as an APFS drive, and then install High Sierra onto it. (Hopefully you would be allowed to do this.). Or clone your Monterey installation onto it – then detach the external drive, erase the Mac, and install High Sierra internally.
After that, you could start up either from the internal SSD or the external one. High Sierra may allow you to run 32-bit applications, but its version of Safari is very old. Even Firefox 115 ESR stopped getting new features some time back, and may not be getting "critical security updates" from Mozilla for too much longer. So you may not want to run High Sierra all of the time - only when you need to for backwards compatibility reasons.
You can upgrade a Mac Pro (Late 2013) to Monterey,, so you may want to upgrade the Catalina installation at some point.
Mac OS will never install older version of MacOS over a newer version.
You don't need to lie about the date.
But what you DO need is to make at least one complete and Trusted Disk-based Backup..
To install an older version, you must completely ERASE your Mac, which also deletes all your stored files. After you do a new install of an older version of macOS, you can deliberately restore your files from your Trusted backup.
Mac Pro 2013 OS Back Date
