Demons are naturally chaotic, so its assumed that without the senior leadership and fear provided by Sargeras, Archimonde, and Kil’jaeden the surviving Legion commanders are battling one another to become the new commander of the Burning Legion. Some may have even left to craft thrir own forces and do their own thing.
The Dreadlords have also seemed to have discarded whatever remains as they returned to Denathrius’ side in Shadowlands.
I personally think that the Burning Legion as an organization is done. But demons themselves are not, for example I can see the Fel Pantheon being used for a demonic force and they could have recruited familiar names off screen.
We could also see some new demonic forces like by familiar faces. For example, Jaraxxus could return leading a force known as the Jaraxxi.