Never assume. Burzzthak the Exalted (Legendary) is not part of the Zorgrom vs Shroud event and is not active, thus why I asked. That LE was not active but was not despawned. This is been rectified.
~ Grim
Confirmed Burzzthak the Exalted (Legendary) finally gone from the map after mentioned it again on Global Chat, although it's easy to verify Burzzthak the Exalted Lvl 15 is in fact part of the Zorgrom vs Shroud event from that event announcement. (I can understand if you said Burzzthak the Corrupter (Legendary) Lvl 5 not part of).
Seems like switching off respawning didn't despawn the already-existed ones on map, which I think it should.
I should have reported this in the Zorgrom vs Shroud IX - General Discussion - Hunted Cow Community instead, so less confusion for everyone.