Security Updates were released for 10.6 and 10.5 (2017-002 and 2017-005 respectively).

Where do you see updates for 10.6 and 10.5? According to https://support.apple.com/en-us/HT208331 - there was a 2017-002 for 10.12 and a 2017-005 for 10.11.

Yes, I did mean 10.12.6 and 10.11.6.

The patch for Meltdown (CVE-2017-5754) for High Sierra, Sierra and El Capitan was contained in these security updates from December 6: https://support.apple.com/en-us/HT208331

Hopefully Sierra and El Capitan will get patched soon.

This statement doesn’t make it clear, but they have been patched (same fixes as High Sierra)

How were we meant to figure this out, through necromancy or something. There is no information indicating Sierra has been patched or is due to be patched.

On https://support.apple.com/en-us/HT208331 there is a reference to CVE-2017-5754 with a footnote saying "Entry added January 4, 2018" (interesting to see this "changelog" being revised as the embargo has been lifted). It says the fix is "Available for: macOS High Sierra 10.13.1, macOS Sierra 10.12.6, OS X El Capitan 10.11.6"

They edited the same page to remove those references. Best to assume Sierra and El Capitan are not patched until proven otherwise.

Clear as mud.