Underworld isn’t permanent to anyone, though.
It was supposed to be replaced by Hero Mastery yesterday but since this mode has been delayed for an additional two days, Underworld should have been extended but at this point, I think no humans are in charge of this, meaning removing Underworld on September 5th was scheduled, by whatever program they use, weeks ago.
Edit : I rememember that sometimes seasonal events were extended (before F2P), especially when something else has been delayed. I guess this time they forgot to extend Underworld until Hero Mastery went live 
…