In years past, when a new version of macOS has come out, the previous one would still be available for download in the Mac App Store’s Purchased tab:
This year, Apple has changed its policy, according to this kbase article:
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.
As Juli Clover at MacRumors points out, it appears that macOS downloads are no longer being tied to a user’s Apple ID. I like this change; I was thought it was a little silly to have to sign in to download a free operating system installer.
However, at least of this writing, macOS Sierra is not in the Mac App Store:
It’s not even available via this direct link that worked earlier this week:
If you want to download the macOS Sierra installer, today may be the last day to do that easily. https://t.co/5JtpzQfJAm
— Stephen Hackett (@ismh) September 25, 2017
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.
While I find it a little annoying, I acknowledge that not everyone is a pack rat like I am. There could be a few reasons for this.
With the APFS transition, going back to Sierra on Macs with internal SSDs is a rather complicated process that requires the removal of all data from the drive. That’s not for the faint of heart.
Moreover, I think Apple considers 10.13 a better, more true form of 10.12. The company wants everyone running Sierra to move to High Sierra. Removing Sierra from download makes that more likely.