• Watch the Apple Event from September 14, 2021 (video)
  
  
• Apple unveils iPhone 13 Pro and iPhone 13 Pro Max (pr)
  
  
• Apple introduces iPhone 13 and iPhone 13 mini (pr)
  
  
• Apple unveils new iPad mini (pr)
  
  
• Apple's most popular iPad delivers even more performance & advanced features (pr)
  
  
• Apple reveals Apple Watch Series 7 (pr)

"This update is recommended for all users and improves the security of macOS."

macOS 11.6 is build 20G165 (11.5.2 was 20G95), so maybe there's other fixes included. You can check the Big Sur forum for feedback, and the Mac Mini, iMac, MacBook Pro, & MacBook Air forums.
FYI: Some macOS 11.4 users said 11.6 didn't appear in software update check and wouldn't install via terminal commands. After installing macOS 11.5.2 from the app store, then 11.6 was available.
(The usual advice on software updates, 3rd party VPN/security software applies.)

Fix Apple DVD Player Not Working on M1 Macs
I (and others) have confirmed that macOS 11.6 broke DVD Playback on M1 Macs.
Tested using Apple Superdrive with DVD movie discs (US/region 1 DVDs).
I had the DVD Player already in the Dock, but if you don't, it's located at:
/System/Library/CoreServices/Applications/
- Select the DVD Player and Get Info on it, then check the "Open using Rosetta" option.
Close the Info window and then drag the DVD Player to the Dock.
I installed Rosetta, and rebooted the M1 mini.
I then ran the DVD Player (from the Dock), and inserted a DVD Movie disc in the Apple SuperDrive. (It played.) Others have also saId the DVD Player running under Rosetta works on M1 Macs with macOS Big Sur 11.6. (Before the macOS 11.6 update, using Rosetta was not required.)

FYI: I still use the above Rosetta tip in macOS 11.7 on a M1 Mini.
(And start the DVD Player before inserting the DVD disc.)

(Another option some have used is the Free VLC Media Player for Macs.)

Article on security fix (revised 9/20/2021)
• About the security content of Security Update 2021-005 Catalina
  
  Download Page for the Security Update
• Download Security Update 2021-005 Catalina (1.45 GB)
  - After installing the macOS Catalina build number is 19H1417.

"* After installing this update, the build number for Safari 14.1.2 is 14611.3.10.1.7 on macOS Mojave and 15611.3.10.1.7 on macOS Catalina."

• About the security content of iOS 14.8 and iPadOS 14.8 (revised 9/20/2021.)
• iOS release notes & iPadOS release notes for 14.8:
  "This update provides important security updates and is recommended for all users."
  
  
• About the security content of watchOS 7.6.2
• watchOS release notes for 7.6.2:
  "This update provides important security updates and is recommended for all users."

• Big Sur forum post that macOS 11.6 broke DVD Movie Disc Playback on his M1 Macs.
  I confirmed after installing macOS 11.6 on a M1, the DVD Player no longer worked.
  See M1 Mac DVD Player Tip (using Rosetta).
  
  
• iTunes U version 3.8.3 (app store link)
  (Available for iOS 12.4 and later or iPadOS 12.4 and later.)
  - This update includes usability and stability improvements.
  - iTunes U will be discontinued at the end of 2021.
• About the security content of iTunes U 3.8.3
  
  
• Pixelmator Pro version 2.1.4 (app store link, macOS 10.14.4 or later)
  "v2.1.4 adds the ability to apply layer styles to RAW layers and includes a few other improvements and fixes." (See linked page for more info.)
  
  
• RIVAGE PM Editor V5.0.1 for Mac - macOS 11 (Intel/Apple silicon), 10.15-10.13
  - See system requirements down the page. (Lists Support for Intel & Apple M1 Macs)
  - Now supports RIVAGE PM Firmware V5.01.

• If you get a personalization error when reinstalling macOS on your Apple M1 chip Mac
  - now goes to If an error occurred while updating or installing macOS
  
  
• Find out which macOS your Mac is using
  
  
• How to prepare your iMac for Apple Trade In

• Microsoft Office for Mac (365/2019) version 16.53.1 (build 21091502)
  - 16.53.1 update Sept. 16, 2021, OneNote update available.
  - See Sept. 14 v16.53 release notes for changes. (Includes security update for Excel.)
  
  
• Microsoft September 2021 Patch Tuesday (isc.sans)
  
  
• Chrome 93.0.4577.82 Stable Channel Update for Desktop includes 11 security fixes.
  
  
• iOS Chrome browser version 93.0.4577.78 (for iOS 13.0 or later)
  - See linked page for changes. (Also read user reviews.)
  
  
• 2021 LG C1/G1/R1 OLED TVs Firmware Update 03.15.95
  - post includes update info and firmware download pages.
  
  
• LG CX/GX/WX OLED TVs Firmware Update 03.23.35
  - post includes info and firmware download pages.