Previously, whenever a major new version of macOS (or OS X) would launch on the Mac App Store, the previous version would still be available for download in the Purchased tab on the store. But things have changed this year. Now that macOS High Sierra has launched, its predecessor, Sierra, is nowhere to be found.
It appears that updates for macOS are no longer tied to your Apple ID, which explains why Sierra and High Sierra don't show up in your Purchased tab. However, Sierra is also missing when you search for it in the Mac App Store.
As Stephen Hackett of 512 Pixels notes:
This means there's no easy way for someone to install macOS Sierra at this point. If your Mac came with 10.12.4 or greater, you can reinstall that version from Apple's servers, but going back from High Sierra isn't doable unless you've got a copy of Sierra laying around.
As Hackett goes on to note, it's likely that at least part of this move is motivated by Apple's desire for Macs running Sierra to upgrade to High Sierra, which offers many refinements of the Sierra experience. It's also worth noting that, at least for Macs with internal SSDs, downgrading from High Sierra would be more difficult thanks to the APFS transition.
