How can data be restored when I downgrade macOS versions?
Copy all important files to another HD (apps, games, your user's library folder, all your other user folders, etc).

Download Mojave, create a USB, boot, erase and install. 

Set up the new computer, then copy stuff over. Some things (like Mail, iCloud, some settings, and iTunes data won't work). If you use steam, it should be pretty easy to copy its folder (within ~/Library/Application Support/Steam to your new user.