The FIDE Laws of Chess, in section 11.3.a, say, "During play the players are forbidden to use any notes".
As for why they forbid notes entirely, as opposed to, say, allowing motivational statements while disallowing the writing down of variations, we can only guess, but I can see two reasons not to allow some notes but disallow others:
There's always going to be some point in the middle of the spectrum where it's ambiguous whether it should be okay or not; and
If your opponent is writing things down, you'd have to either take his word that everything he was writing was legal, or summon an arbiter and demand to see it. Neither alternative is that appetizing.
By the way, So's notes were arguably chess-related in that they addressed things like time usage.