Make Cleanse remove all debuffs including knock-ups instead of only CCs

How about no? That's completly broken.

I think the reason that knockups aren't cleansed is that from an animation perspective, a champion suddenly no longer being airborne is hard to do. The champions with knockups are then balanced around their CC being unstoppable, if it lands.

it does actually remove ignite.

id say cancelling all forms of cc would be fine. though.

Suppression is meant to be difficult to stop, Riot has gone on record saying they would sooner make it COMPLETELY impossible to stop (meaning no QSS either, you'd need to stop the caster themselves via an ally helping you or something), and then balance the skills that apply it from there.

Can't see that working out well for knockups either since you would BASICALLY teleport onto the ground from the air, would look really wonky animation wise.