If you decide macOS High Sierra isn’t for you and want to reinstall Sierra from the App Store, good luck because it isn’t there. Checking your purchases list won’t help, either, because macOS Sierra seems to be completely MIA.
Searching for macOS Sierra on the Mac App Store returns a few results, but none are Apple’s operating system. Looking in your purchased apps gets you a big bucket of nope even though older Mac operating systems show up.
According to Apple’s knowledge base article on downloading previous purchases, which includes older operating system versions, “macOS Sierra or later doesn’t appear in the Purchased tab. Instead, use the Search field in the upper-right corner to find and open the App Store page for that macOS.”
That’s all well and good, except that you can’t find macOS Sierra. It’s likely this is an intentional move because a substantial number of High Sierra installations will have transitioned from HFS+ to APFS for their boot volume format and you can’t roll back without reformatting.
This is a big change since Apple has previously left old operating system versions on the App Store. Changing that policy now will no doubt be a major pain in the backside for anyone who prematurely made the jump to High Sierra, like people who later find they’re having serious app compatibility issues.
Since Apple deems macOS Sierra downloads verboten—at least for now—the only way you’re getting it back is if you saved the installer at some point, or have a good backup.
[Thanks to 512 Pixels for the heads up]