I have downloaded and installed all Mac OS X versions since Mac OS X 10.4 or earlier (as well as all classic Mac OS versions!).I recently checked, and Sierra 10.12, High Sierra 10.13 and Mojave10.14 full installers were not showing up under my Apple ID in the Mac App Store (MAS) under Purchases, despite the fact I had downloaded them at the times they were released. I have those installers archived on backup external drives. ... Curiously, my Apple ID does show all earlier OS X installers Lion, Mountain Lion, Mavericks, Yosemite and El Capitan under my Apple ID/Purchases. All (but Yosemite) had actually been installed historically on the MacBook Pro internal drive and each in turn on top of the previous version.
Running macOS 10.12.6 Sierra, the Mac App Store is showing, under Purchased, OS X 10.7 through OS X 10.11 with Download links. macOS 10.12 and later do not appear there.
Running macOS 10.14.1 Mojave, no earlier macOS or OS X versions are shown in the Mac App Store (only OS X Lion Server and macOS Server).