So for those who weren’t aware, Onslaught replaces purified debuffs with a passive version but ever since his release this has only worked against purify but NOT cleanse. This is contrary to how every other similar champ has worked (Bishop, Dust, Storm X, etc.) where abilities that punish purify in some manner also punish cleanse (note that this is different from abilities that
prevent purify like Apoc, which does not work against cleanse because cleanse is still a different ability from purify). However, as of now this is no longer the case. I had to use Onslaught against a Chee’ilth in war just now for lack of better options and found myself getting my passive stuns and daunteds. Here’s a practice fight against Zemo demonstrating it:

Not sure exactly when this was fixed, but I found no patch note addressing this that I missed nor a comment/thread pointing out it’s been fixed so I’m putting it out there that he works properly now.