Wayland uses XKB, as does everything else under the sun. XKB is the layer that implements the Compose Key (Actually, you need to configure it via XKB because other methods like ibus do not bother reading ~/.XCompose which allows you to set your own custom compose sequences)
> On Xorg the default Compose Key is ⇧ Shift+AltGr [...] The X keyboard driver does not allow the key used for Compose to also function as a modifier.
Which might explain why my altgr doesn't seem to work when I try it since it's mapped to mod in i3.