...is out for download. I've read this might be the last update to Sierra before High Sierra later this year. (This update also updates iTunes to 12.6.2) ...Running smoothly.

I updated through Mac App Store (via Apple menu).

iOS, watchOS, & tvOS updates are also available.

For those wanting to upgrade using the Combo, here's the Link

Ditto, and all is well.

macOS 10.12.6 also includes a Safari update to v 10.1.2.

Given High Sierra is already on its second PUBLIC (fourth DEVELOPER) beta it is surprising that Apple is still devoting any effort to Sierra. Certainly application developers are going to be focused on High Sierra compatibility. The same can be said for iOS 10 and Watch OS 3.

From various sources, it appears all the updates are mostly correcting security vulnerabilities. ...So, it's good Apple isn't overlooking Sierra (as well as Safari & iTunes), along with iOS, watchOS, & tvOS, given major upgrades are on the horizon.

I know you're an early adopter, but finding it surprising that the current operating system is still being supported is a bit of a stretch, no?

In the past, Apple has only really done any updates for the one prior release. But given they've better than doubled the speed of their new releases, it's not surprising they're supporting a little farther back now. Same amount of time maybe, just seems longer is all because the numbers are growing so much faster.

Apparently I didn't express myself as well as I thought I had. I do not find it at all surprising that Apple is still supporting all of their current OS versions including Sierra, iOS 10, WatchOS 3, and TVOS ?? - in fact I would be appalled if they didn't. However, given the current state of development of High Sierra, et. al. and the current level of effort being devoted to the next round of upgrades I would be surprised if there were any new releases of the current versions other than a security patch/update if a vulnerability should appear. For anything else it would seem to make more engineering and economic sense to push improvements into the forthcoming upgrades rather than continuing two not quite parallel development forks running.

I'm still 10.11.6 and yet Safari has been updated to 10.1.2

It probably came along with Security Update 2017-003 (El Capitan).