So I love that ignite got a buff. It was admittedly undertuned before, and I like the extra diversity it brings to top/mid lane where TP was running rampant. Having said that I feel like Exhaust is essentially in the same boat that Ignite was before, where it's just so weak there is not a reason to take it. It's not as useful for getting kills as ignite it, which is as it should be, so that's fine. But it's also not as good at denying kills either.
From a flavor/design standpoint there is an additional issue of no longer having dedicated support-oriented summoner spells that are viable. Heal is there, sure, but ideally you want your carry to take heal so they can get the max value, it's inefficient to have the support take it. At any given time there needs to be at least 1 summoner spell that does supporty things and is optimal to be carried by the support. So again, Exhaust needs love.
Here's my 3 recommended changes:
1 - extend duration from 2.5 to 3 seconds
One of the issues with exhaust is that most of the characters you want to use it on are mobile divers. So if you're bot lane and you have like a Zed pop in, it's too easy for him to pop back out and then try again in a couple seconds. Extending the duration slightly makes the chance of a retreat before a re-engage a possibility.
2 - Increase % damage reduction from 40% to 50%
The attack speed reduction here is good, but it make Exhaust optimal against fighters and ADC's who generally are not doing as much of that up front damage early game. So increasing the % damage will help against, the Kats-Zeds-Lees-Khas of the world in the early game. Additionally it helps with the scaling a bit, which I think is nice. Riot has made a lot of changes recently that de-empasize having different phases of the game (early-mid-late) and really kind of close the scaling gaps. Personally I think that's a bit of a mistake and rather than changing things like stats, champions, and runes, I would prefer these changes come through on summoner spells and item use functions so they have that skill orientation of a cooldown and knowing when to use. So for instance, rather than nerfing Jax's late game scaling, you just buff exhaust so that it actually still gimps him at lvl 18 and 6 items. Now there is a window for both Jax and his opponents to track and play around.
3 - Increase range from 650 to 675
This isn't that big of a change, but I think one that is necessary. As something like an enchanter you're goin to be in the back line, also trying to avoid getting gibbed with your carries. So being able to pop Exhaust onto the divers before they get to you is a must. And I often play stuff like Alistar or Braum as a bodyguard for my backline rather than a vanguard frontliner. So a lot of the time the divers are quite literally on top of the carries before I touch them. Being able to pop Exhaust as you see them coming in would make the backline tank support an even more viable option, considering that ignite is far less useful in the backline.