Jim Dalrymple of The Loop was told by Apple that beginning today, Macs with automatic downloads turned on for the Mac App Store will auto-download updates of macOS Sierra. As Jim emphasizes though,

It’s important to note that this is not an automatic installer—this process will only download the update in the background, and then alert you that it is available to install. You can choose to install it when its convenient. You can also choose to ignore the update.

In addition, Sierra updates will not auto-download if your Mac doesn’t support the update or doesn’t have enough free storage.

Automatic OS updates make a lot of sense as a way to encourage adoption of the latest version of macOS and should help spread critical security updates more quickly. The change is not without its downsides though. For instance, updates can be large, which could be an issue for people with limited data plans as Stephen Hackett points out on 512 Pixels. Notwithstanding that sort of issue, I think the change strikes a good balance between automation and requiring intentional user interaction to update macOS, which should work well for most users.