Can I use an imac late 2013 as a display for a mac mini 2020?

If that Late 2013 iMac is a 27 inch model running nothing later than High Sierra (10.13.6), and you have connected that 2020 Mac mini to the iMac with a TB3 to TB2 cable, and then a TB1/2 cable to the iMac.

BTW, you can set it up as dual boot to High Sierra or older

and it will still work. Just boot into the lower version of

macOS and proceed. I just did a verification with my late 2013 27"

iMac which has Catalina installed but I have an external Sierra

volume that I still need to make use of. Booted to Sierra, Command-F2,

and off I go. I did the test with an older MacBook Pro but it shouldn't

matter except for the adapter chain.