I've also seen people "nod no"
These days, "shaking (their) head" defaults to "no"/"uh-uh"/"nuh-uh" for me, with exceptions including "shaking (their) head (sadly) as (they) exclaim/explain that..."
"Nodding" on the other hand always implies "affirmative" in some way, even if it's accompanying a "no, yeah" or sometimes affirming a negative - nodding (unless it's "nodding off") is always a "yeah" in some way.
SHAMEFULLY, I have to admit:
I was almost 19 years old when I first learned that "shaking (their) head" referred to the side-to-side "nuh-uh" motion - I knew what the motion WAS, I didn't know what people CALLED it - and so every time I would run across "shaking (their) head" in text or audio I assumed it meant someone either Jacobs-Laddering their head around wildly, or removing their head and shaking it like a magic 8-ball.
NO ONE TOLD ME.