To me it just feels pointless compared to ignite and there are many reasons it feels like ignite easily outclasses exhaust.
1. Shorter cooldown: It’s free damage you get to use more often.
2. Slow from exhaust feels useless: When so many characters can just ignore the slows with their 20 dashes it just doesn’t feel like it doesn’t effect them.
3. Damage reduction feels pointless: When everyone can get bursted down super fast, even with their damage being reduced by 40% it’s just feels like why bother.
4. Ignite has Grevious Wounds: Right now it seems like the most problematic champions in the late game are those with either hyper mobility or nonstop heals. Exhaust can’t stop mobility but ignite can at least reduce healing. Espically since no one wants to get healing reduction. It’s clearly way more important to get that second infinity edge Jhin than stopping Illaoi/Aatrox from healing back to full hp.
5. Burst or be bursted: When everyone can blow up each other super fast and one kill early game means you lose the entire laning phase, it just feels way more important to make sure you get that first blood, even if it means kill stealing as a support.