Where on disk does the macOS system-wide dictionary store the list of words I added?

For those attempting to find the Unix standard words file, it's at /usr/share/dict/words.

Are you certain you really mean system-wide? If I add a custom spelling, in Snow, it seems to end up here:

~/Library/Spelling/LocalDictionary

The System-wide equivalent, which is empty on my Mac, would be here:

/Library/Spelling/

Aside from this, you will find packaged dictionaries here:

/Library/Dictionaries/
~/Library/Dictionaries/