This should still work:
get_tree().get_root()
However, the real root is the root viewport (what you see on screen). So your "World" node would be the first child. So you could also do this, but it's not really less code.
@onready var player = get_tree().get_root().get_node("World/Player")
Juan is working on fixing this with adding unique names, but it is still under discussion, so who knows when that will be added. But with the new syntax (which does not exist yet) you could do this anywhere from any script.
$@Player.position.x += 1.0