I researched through several reference books and modules to put together everything I could find about the exhaustion condition in D&D 5e. I hope this resource will help you in your games! 

There is a spoiler for one monster in Tomb of Annihilation, so be warned. 

https://dungeonswithdragons.blog/2023/08/26/an-in-depth-look-at-exhaustion-for-dd-5e/

If I missed anything, please let me know and I will update the article to include that info.

 Thanks!

nice. Well Done. It would pretty hard, or a dumb character trying very hard, to die from exhaustion.

once my drow cleric died from exhaustion because i had nightmares about spiders

Quote from gnomeman555 >>

once my drow cleric died from exhaustion because i had nightmares about spiders

 Like...all the levels of exhaustion? How'd that happen!? 

Quote from Journalismist >>

Quote from gnomeman555 >>

once my drow cleric died from exhaustion because i had nightmares about spiders

 Like...all the levels of exhaustion? How'd that happen!? 

 Really bad con saves

They changed how exhaustion works in 2024  https://www.dndbeyond.com/sources/dnd/free-rules/rules-glossary#ExhaustionCondition

Exhaustion [Condition]

While you have the Exhaustion condition, you experience the following effects.

Exhaustion Levels. This condition is cumulative. Each time you receive it, you gain 1 Exhaustion level. You die if your Exhaustion level is 6.

D20 Tests Affected. When you make a D20 Test, the roll is reduced by 2 times your Exhaustion level.

Speed Reduced. Your Speed is reduced by a number of feet equal to 5 times your Exhaustion level.

Removing Exhaustion Levels. Finishing a Long Rest removes 1 of your Exhaustion levels. When your Exhaustion level reaches 0, the condition ends.

 

Unrelated the sibriex is one of my favorite creatures that causes exhaustion. Probably one of the nastiest exhaustion effects especially with the flesh warping variant that adds random mutations to it. 

Warp Creature. The sibriex targets up to three creatures it can see within 120 feet of it. Each target must make a DC 20 Constitution saving throw. On a successful save, a creature becomes immune to this sibriex’s Warp Creature. On a failed save, the target is poisoned, which causes it to also gain 1 level of exhaustion. While poisoned in this way, the target must repeat the saving throw at the start of each of its turns. Three successful saves against the poison end it, and ending the poison removes any levels of exhaustion caused by it. Each failed save causes the target to gain another level of exhaustion. Once the target reaches 6 levels of exhaustion, it dies and instantly transforms into a living manes (see the Monster Manual) under the sibriex’s control. The transformation of the body can be undone only by a wish spell.