It's obviously Apple's right to change their policy whenever they want, but considering that in my Mac App Store "Purchased" list, every version of OS X from Lion through El Capitan is available for download, it seems reasonable for some of us to be upset that Sierra is missing, especially when out of FIVE Apple Advisors I chatted with or spoke to yesterday (one of whom was a "Senior Advisor")
none of them was aware of this as an official policy change.
And given that there's a direct download page for El Capitan, it seems reasonable to expect one for Sierra.
https://support.apple.com/en-us/HT206886
Also: Sierra should be available for re-install via Recovery mode for those who already have it. But I've tried multiple times over three days now, and get the error message "This item is temporarily unavailable. Try again later." None of the support people had any idea why this is happening.
The support document on re-installing purchases says to re-install Sierra, just search for it in the App Store app. Either the article is wrong, or something is wrong in the App Store.
https://support.apple.com/en-us/HT201272