Neither macOS Sierra nor macOS High Sierra show up in the Purchased list, suggesting the updates are no longer tied to an Apple ID account. Previous Mac software updates were linked to an Apple ID account and updating required an Apple ID and password, something that could be a hassle when a Mac changed ownership.
An Apple support document on reinstalling apps confirms that the change to remove macOS Sierra and High Sierra from the Purchased tab was intentional.
"macOS Sierra or later doesn't appear in the Purchased tab," reads the document.
In the case of macOS Sierra, the change means that there's no way for Mac users to download macOS Sierra should they want to downgrade from High Sierra for some reason.
OS X El Capitan, OS X Yosemite, OS X Mavericks, and earlier updates are all tied to a user account and listed in the Mac App Store. Apple has also made a link to the OS X El Capitan update available through a support document, but no similar support document is available for macOS Sierra.