Bringing Hexy Back

To do this in Shadowlands, I suggest bringing a level 50 character to be able to be hexed (as level 60 players will 99% deflect it). When the level 50 player has received the debuff, have two players stay on top of eachother all the time. once the debuff is actually cast on someone it will jump between players without failing.

Throughout the dungeon, exclusively on the right wing (the trash on Vol'kaal's side) there are mobs called Zanchuli Witch Doctors which cast, among other things, a hex on a target that lasts 5 seconds. While hexed, you maintain control of your character and can use a variety of abilities, though not most of your attacks. You get a circle around you which, upon expiration of the hex, applies the hex to anyone within your circle.

You need to bounce this hex back and forth between two people for the entirety of each boss fight. You don't need to do all 4 bosses in the same run, and you don't need to use the same Unstable Hex for all 4 bosses, as there are multiple Witch Doctors. I'd personally recommend clearing all the trash you can to Alun'za, doing her and Rezan in any order with the hexes, then click the levers to get the access through the middle to Vol'kaal, then using the Hex Priests just before his room during his fight, and pulling another Hex Priest for Yazma.

As far as fight-specific advice goes, this is all I got:

Rezan - Make sure the people passing the hex have a pre-assigned spot to hide from the fear, and have everyone else go somewhere else to avoid being hexed. If either of the hex passers gets fixated, make sure they both run to continue passing the hex.
Alun'za - Both hex passers need to be collecting blood pools for Transfusion, so make sure you leave early if you have to in order for both of you to run to two different puddles. Assigning you hex passers to two nearby puddles will help with this.
Vol'kaal - As a tank you do virtually nothing on this boss until the totems die, so you can safely pass the hex to a tank for P1 to allow DPS more uptime on the totems. Otherwise just make sure they die evenly and continue to pass the hex between two people.
Yazma - The Soulrend is the only worrying part, because you won't be able to get the adds 100% stacked unless you get some lucky timing with the hex, which probably isn't worth the risk. Instead, have you hex passers go a little bit behind your non hex passers so that all 4 adds still travel in a path towards the boss. In either case they die very quickly.

If you run out of Witch Doctors, just run out and reset the dungeon, they'll respawn.

My btag is FrostyShot#1667 if you have any questions about the metas. (US Servers)

As of Shadowlands level 60, it is possible (though a little annoying) to solo this as a hunter. I pulled 5 Witch Doctors for higher probability of the hex landing, kited them to each boss with me, DPSed boss down to 5%, then waited for a hex cast to land (it still missed a lot). Once I was frogged, I commanded my pet to kill the boss before the 5 seconds of hex expired. If the pet wasn't going to kill the boss in time, I set my pet to passive and waited for the next hex. I used the same Witch Doctors from boss to boss by feigning death to make them reset. Good luck!