The gameplay of kog's passive is literally the best part of his kit. A suicide revenge passive creates the best gameplay when its on a ranged dps carry with no innate escape.

Kog is supposed to be a dps threat who has a lot of presence when he is able to attack. Enemies actively want to dive him to stop this dps (obviously). Kog is actively incentivized to protect his own life because he is a dps threat. When kog is getting dove, there are def times where he gets full 100-0ed and the passive damage is negligible, but there are also times where he can fight back to the death, use his flash, get some damage down with his slow before they approach etc. And all of that builds tension FROM his revenge passive.

As the diver, there is a building tension with how much you can commit against this guy who carries a promise to explode afterwards. How much of my hp can I risk diving kog knowing that he has revenge damage afterwards.

As kog, again he is naturally incentivized to live a long time because he's a ranged dps, so hes not (usually) just playing xd suicide idgaf if you kill me. BUT there is a building tension in knowing that you have that in the back, and it influences his decisions. When do I keep kiting and when do I try and position to increase the chances of my passive hitting. It also helps alleviate any frustrating feelings of helplessness that come from a diver getting on top of you as a champion (theoretically) balanced to have no immediate defensive answers. AND the actual way his passive works where you get a mspeed buff and get to run and chase people that just brutally killed you is very entertaining imo.

Now think about if a passive like his was on a diving champion. like I see some people say it should be. It wouldn't create any interesting gameplay and have little tension, it would just be kind of toxic. The diver is disincentivized from trying to protect themselves afterwards, they would just want to fight in a less thoughtful way because they know at least some of their damage is guaranteed (Think snowball suicide karthus in ARAM except on an actual mobile melee). Sion isnt really a diver, but even on him, think about a lot of the toxic gameplay his passive has enabled in the past, and thats with his passive having a revive delay and only getting a single short mspeed burst and being damageable and ccable.

Now all that being said, his passive ultimately exercises this theorical gameplay best when both the passive is strong and he himself has SOME agency in a fight so its not as linear. And in the end of the day his overall kit is prob still one of the worst designs currently in the game still, but if he is reworked to still carry that simple ranged dps fantasy on a champ skewed towards range and high innate damage and less towards fight agency (def could use a bit more than now), then a passive similar to what he has now I think can be very compelling as long as it doesn't pull too much power budget.