I would like to know where I can find a list of all possible defaults write terminal commands. (Core OS X related commands, not any due to user installed software.)

Apple themselves must have a manual or technical specification for OS X operating systems and such commands.

Here is an example command (which doesn't work in El Capitan by the way) which demonstrates

defaults write com.apple.driver.AppleBluetoothMultitouch.mouse MouseHorizontalScroll -bool NO

I assume such commands simple write to various configuration files hidden in the OS. It would alternatively be useful to find a document describing all such files and their location on /.

Solution (which was posted in the comments) is defaults read

Solution 1:

This is like asking for a list of all possible websites or all possible registry settings. These commands write to Apple and non-Apple software, and will vary depending on the apps/services installed and their versions.

Since this isn't a single unified settings database, I wouldn't assume Apple maintains a single document or manual on the settings.

Solution 2:

Things like this aren't always documented. I've seen keys change between minor versions as well as major ones, which adds to the difficulty in creating/maintaining such a document.

In this case, the key you're using MouseHorizontalScroll is still correct it's not working in the latest version. :( I've heard this is working in the latest seeds of macOS Sierra, though.