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.
I have heard of organists using music oriented this way. It does have some major advantages for keyboard players, and I have sometimes considered using it when teaching students who have difficulty doing the 90 degree turn in their brains when orienting the notes moving on the staff and their hands/fingers moving.
But as you say, it’s not great for other instruments, and in the long term, would be difficult to make work in the larger musical world.