How to know the macOS version used for most recent clean install?
I've Looked for clues in /Library/Receipts/InstallHistory.plist

On a few computers I checked, they typically start with a "com.apple.pkg.OSInstall" package. It doesn't mention OS version, but scrolling down to most recently applied OS updates, and it may become clear what the OS version was by the updates.

BTW, all bets are off if the drive was cloned from another computer.