I discovered the existence of Klavarskribo (“keyboard writing” in Esperanto). It’s a development of musical notation intended to make playing keyboard music easier. It turns the staff 90 degrees clockwise, so the music is read top-to-bottom, with higher notes to the right and lower notes to the left, as on a keyboard. It uses a chromatic staff, so it doesn’t need accidentals.
I’m sure that for someone without prior knowledge of the usual notation it would be just as easy to learn to play using this notation. However, I don’t see that it has any advantages for voice or instruments without keyboards. It has one huge disadvantage in that included lyrics would have to be read either sideways or word-by-word down the page. So I don’t think it makes very much sense for anything but keyboard music, and if you want to play or sing anything else, you’ll need to learn the usual notation anyway, so I can see why it never really caught on.