Fairly taking a stance | Golf Monthly

The "leaves" examples were part of an old decision pre-2019 (excerpt below), the principles of which are unchanged, as Rulie notes.

Rule 13-2 prohibits a player from improving certain areas. What does
“improve” mean?
A. In the context of Rule 13-2, “improve” means to change for the better
so that the player creates a potential advantage with respect to the position
or lie of his ball, the area of his intended stance or swing, his line of play or
a reasonable extension of that line beyond the hole, or the area in which he
is to drop or place a ball. Therefore, merely changing an area protected by
Rule 13-2 will not be a breach of Rule 13-2 unless it creates such a potential
advantage for the player in his play.
Examples of changes that are unlikely to create such a potential advantage
are if a player:...........

• accidentally knocks down several leaves from a tree in his area of
intended swing with a practice swing, but there are still so many leaves
or branches remaining that the area of intended swing has not been
materially affected; ........

Examples of changes that are likely to create such a potential advantage
are if a player: ......

• accidentally knocks down a single leaf from a tree in his area of intended
swing with a practice swing, but, as this was one of very few leaves that
might either interfere with his swing or fall and thereby distract him, the
area of intended swing has been materially affected;