I imagine that this is a very common request because a lot of people are frustrated that they can no longer run their 32 bit apps once they upgrade to Catalina, and it isn't easy to downgrade.
I am not too picky about which version I install, I picked High Sierra just because it is the most recent version in the pull-down in VirtualBox when creating a new VM. I wish to install an older version of Mac OS in a VirtualBox so that I can run some 32 bit apps since Apple eliminated support for them in Catalina. I have a MacBook Pro running Mac OS X Catalina 10.15.3. For what it's worth I have the same issue with VirtualBox 6.1.8.
