The Burning Legion created the Scourge and the Lich King, right? So wouldn’t the defeat of Sargeras and the Burning Legion have destroyed the Scourge, since their power was from the Burning Legion?
Not quite.
The Legion did engineer the original Scourge and the plague of undeath. But once it’s been released, it’s out there.
The Scourge haven’t been under the direct control of the Legion in longer than I can remember. But they were still a thing operating independently (so to speak)
It’s kind of like the original Green orcs or the red Fel Orcs. Mannoroth and Magtheridon are long since dead, but the Azerothan orcs are still green and the Draenoran Fel orcs are still red, as are their children with non-Mag’har orcs. Much like the Man’ari will likely always be red even though they are no longer under the influence of the Legion.
Many of the Broken have been away from the corruption of Outland for decades at this point, but their bodies haven’t reverted back to normal Draneai/Erradar.