There are a couple sections, but for the most part it's chronological, with new entries at the bottom. Started adding dates to new notes/quotes/observations about 5 years ago.

It's a sprawling mess, so I rely on search to find what I'm after.

I reference it often -- it's like an extension of my brain.

I do the exact same thing. Just a text file that these days I store in a Dropbox folder so that no matter what system I'm on I can just "vi ~/Dropbox/notes.txt".

Similarly, several years ago I added a keybind to my .vimrc that, by typing ",a" I'm moved to the bottom of the file and today's date with a separator line underneath. My leader key is "," and I mnemonically associate "a" with "append".

The earliest date recorded in it goes back to 2006 and is about 300KB in size. I still reference and add to it on a regular basis.