I wonder if everyone means the same thing by "Clean Install"?
I think most people do it to get rid of cruft and remnants.
Remember the Signed Sealed System Volume cannot be anything but "Clean" and unmodified, because it is extremely rigorously checked every boot and will not boot if it does not pass. The SSSV does not have any cruft in, so not point in erasing the System Volume before upgrading.
Absolutely all cruft and remnants exist on the -Data volume so a clean install would be
Erase all Content and Settings to put it back to virgin factory state, then re-installing all apps, serial numbers, resetting all system and app preferences, and reloading data. Anything involving migrating brings the cruft back. I do this about once every ten years.
No need to use Disk Utility for a Clean Install.