Requested in #2900 (comment).

Should be possible from an API perspective, though might be a bit tricky to change in the backends.

Platforms that just use PhantomData<T> in the type (and thus don't need the actual user event type):

• Wayland
• X11
• iOS
• Android
• macOS
• Orbital: Make EventLoopWindowTarget independent of the user type on Redox/Orbital #3055
• Web: Web: Remove T from EventLoopTargetWindow #3081
• Windows: Windows: Make EventLoopWindowTarget independent of UserEvent type #3061

Finish:

• Remove from top-level type: Remove generic parameter T from EventLoopWindowTarget #3298