So, to facilitate Macs changing hands, Apple has un-facilitated d/l'ing macOS; their's either something seriously wrong with their logic, or they're "crazy as a fox". tongue
Other posts have stated this, but Recovery Boot Mode provides for ways to install older and current systems, perhaps obviating the need to archive purchases in the App Store:Command-R
Reinstall the latest macOS that was installed on your Mac, without upgrading to a later version.1Option-Command-R
Upgrade to the latest macOS that is compatible with your Mac.2Shift-Option-Command-R
Requires macOS Sierra 10.12.4 or later
Reinstall the macOS that came with your Mac, or the version closest to it that is still available.