10.12 is only available from the App Store if you have already downloaded them before. They are in your purchased section of the App Store if you need to download them again. High Sierra, which has the same system requirements is still available (as of 9/30/2018):
https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12
If you have an older Mac and wonder if you can upgrade to Sierra or high Sierra, I have posted this tip as well.
Which 10.6 Macs can install Mac OS Sierra and High Sierra
These tips all refer to the client version. If you are using Mac OS X server, please check: Server forums
for more posts on Mac OS X Server.
On September 25, 2017, Apple released 10.13 High Sierra for Macs. It has the same minimum requirements as its predecessor, except that it needs to be updated from 10.8. However, if you want support for Apple's new hardware acceleration, you will need:
iMac: late 2015 27" or newer, 2017 21.5" or newer
MacBook: early 2016 or newer
MacBook Pro: late 2016 or newer
Backing up your data before installation as always is a must.
A new file system has been introduced with 10.13, known as APFS:
How to choose between APFS and Mac OS Extended when formatting a disk for Mac - Apple Support
discusses when to use it. A tip I wrote earlier discusses the advantages of other file systems as well:
http://discussions.apple.com/docs/DOC-3003
RoaringApps continues to provide a third party listing that when updated past the release date is fairly reliable for the operating system. Please update it if you find inconsistencies.
The forum macOS High Sierra from Apple offers user to user Apple moderated discussion about the unique features of the operating system.
Other user tip authors are welcome to contribute any specific updates.
10.13.1 was discovered to have a serious root bug which was eventually fixed with 10.13.2.
10.13.2 was released December 7, 2017
10.13.3 was released January 23, 2018
10.13.4 was released March 29, 2018
10.13.5 was released June 1, 2018
10.13.6 was released July 9, 2018
Safari 12.0.1 was released on October 31st for both Sierra and High Sierra through the App Store.
Security Update 2020-001 was issued January 28, 2020.
Some applications may need to run with System Integrity Protection disabled. See this tip for details:
https://discussions.apple.com/docs/DOC-250002177
Unsupported Macs may have High Sierra installed by the tool from http://dosdude1.com/highsierra/
Use at your owm risk.