Prior to now, on every occasion a big new model of macOS (or OS X) would release on the Mac App Store, the earlier model would nonetheless be to be had for obtain in the Bought tab on the retailer. However issues have modified this yr. Now that macOS Prime Sierra has introduced, its predecessor, Sierra, is nowhere to be discovered.
It seems that that updates for macOS are now not tied in your Apple ID, and is the reason why Sierra and Prime Sierra do not display up to your Bought tab. Then again, Sierra could also be lacking while you seek for it in the Mac App Store.
As Stephen Hackett of 512 Pixels notes:
This implies there is no simple means for anyone to put in macOS Sierra at this level. In case your Mac got here with 10.12.4 or higher, you'll reinstall that model from Apple's servers, however going again from Prime Sierra is not possible except you have got a duplicate of Sierra laying round.
As Hackett is going on to notice, it is most probably that no less than a part of this transfer is motivated by way of Apple's want for Macs operating Sierra to improve to Prime Sierra, which provides many refinements of the Sierra revel in. It is also value noting that, no less than for Macs with inside SSDs, downgrading from Prime Sierra can be tougher because of the APFS transition.