We do not know the currently state of Zandalar, we just know that the huge isle has suffered the cataclysm like the rest of Azeroth. But thinking that Zandalar has completely disappeared is venturing too far.
In fact we only know the version of that prophet Zul...
Indeed, until Kezan, that suffered the Cataclysm in an apocalyptic form, has not been destroyed and we can visit it in the near future, whose sovereignty will be of the Horde, as is right.
Zandalar will rise again! The cradle of the troll civilization will rise from its ashes if necessary... the Zandalari will not surrender, King Rastakhan won't allow it! Zuldazar will shine with light itself!
The Zandalari are being ruled by a despot, a visionary who has annulled the reason of the King Rastakhan in order to direct himself the fate of the Zandalari. That prophet Zul wants to restore the troll supremacy in Azeroth, his own supremacy, and for it he has betrayed the confidence the King placed in him, and now leads the sons of Zandalar to ruin!
But all is not lost ... someone can prevent that the last troll tribe ends up like the others: mired in corruption and self-destruction. And that is the new leader of the Horde, Vol'jin.
Vol'jin will get overthrow that prophet traitor and will end his dream of greatness. Only he can return reason to the wise King Rastakhan and earn the respect and admiration of his troll brothers.
The feat of Vol'jin will serve as inspiration to the Zandalari, and they will see that for not extinct and for that their civilization is not diluted in the sands of time they have only one option: the Horde. The Zandalari will see that only as full members of the Horde they will have the capacity to be present in the decisions that will shape the future of Azeroth.
In addition, as a fundamental part of the new Horde of Vol'jin, the Zandalari have full support of all the races of the Horde to rehabilitate their home, Zandalar. For their part, King Rastakhan will offer full support to the Horde, both his ancient knowledge and as their powerful military and naval force.
Moreover, because the Vol'jin's obligations as Warchief of the Horde will prevent him taking over the Darkspear tribe, the Darkspear will be integrated into Zandalari society, and will make Zandalar their home, Zuldazar their capital, and Rastakhan their king, albeit they will continue showing all due respect to the Warchief Vol'jin in particular and the rest of the Horde in general.
By an Horde more united and stronger! Lok'tar ogar comrades!