There's PSK and the likes which mess with phase but I don't think that's the same as what you're asking as you would typically then use something else to actually get separate streams with it. The problem with pure and plain phase division multiplexing is how do you separate it back out on the other side? You can make up as many pairs of wave values to create the resulting wave without doing something else beyond plain multiple phases.

Time division multiplexing and frequency or amplitude division multiplexing preclude most things because they are so cheap and simple these days. Polarity is also another knob to mess with when you need to squeeze more.