[limeblack]

If you have an Apple store near by they'll set it up for dual boot. If you don't mind do it yourself partition and install the installer to thumb drive.

I don't condone piracy but you can download the old installers(get creative) md5 checksum it and simply reinstall the old OS. I am still running a 2012 macbook pro for video editing with final cut pro.

[officeplant]

You don't really have to pirate anything. Apple provides a guide for creating USB installers. Used it to bring my 2012 Mini back to Mojave recently.

https://support.apple.com/en-us/HT201372