Taking notes at Comp Rel sealed
No, it's not specifically prohibited; perhaps because it happens so rarely?
All the same, there's not a lot of value in those notes. They can't be shown to anyone during the deck build, so they can't influence deck build choices. They can't be consulted during games. The only real value is in mulligan decisions (very limited application) and, occasionally, sideboarding decisions.
If it becomes problematic, we will consider addressing it.
d:^D