I have played all the halo campaigns besides halo wars and I can’t remember what happened to the brutes. I loved fighting the brutes on legendary because it got your heart pumping whenever one would go enraged and if I had to chose to fight either the brutes or the prometheans I would pick the brutes any day. If you know what happened to the brutes and if they could make a return please let me know as I would love to use the brutes weapons again in halo 6 
Bad economy, couldn’t afford to pay them.
On a serious note; I know parts of the Sangheili economy used Jiralhanae workers, but as foot soldiers im not so sure. There are hostile Jiralhanae factions out there still though, just got done reading “Last Light” (great book by the way) that had one of those factions in it. A religious one that worships the Forerunner just like old school covies did. Jiralhanae weren’t the only species in the faction (also had humans) but they seemed to be the main hierarchy
Brutes are around, they just have reverted back to their tribe-ish mentality for the most part. They have somewhat of an uneasy peace with the Elites and they are scattered throughout the galaxy, doing BRUTEISH things. (haha) Anyway, I know it’s not super detailed, but I hope this sheds some light.
Hopefully they make a return in the next Halo, as well as Drones. I miss those annoying flying buggers.
If there are Gravity Hammers, there should be Brutes, too.