Someone made it really easy to do just Download.
Ensure you have these programs available on your Linux distribution: unzip, wget, basename, dotnet, md5sum (the only one you are likely not to have is dotnet; you can install it with
snap install dotnet-sdk --classic --channel=latest/edge
Open HorizonZeroDawnDowngradeScript.sh with an edit programs like notepadqq.
And edit the top line.
Open the terminal and say CD (link to HZDDowngrader-1.1 Folder) and push enter.
now put this command into the terminal and push enter.
You will be prompted for Steam username and password as well as the 2FA code two times.
Then it will do the rest for you.