You can even run the older version in a VM, with restrictions. https://www.apple.com/legal/sla/docs/macOS1012.pdf:

”you are granted a limited, non- transferable, non-exclusive license:

[…]

iii) to install, use and run up to two (2) additional copies or instances of the Apple Software within virtual operating system environments on each Mac Computer you own or control that is already running the Apple Software, for purposes of: (a) software development; (b) testing during software development; (c) using macOS Server; or (d) personal, non-commercial use.”