The reef still exists, and it should still be led by Petra Venj, as we have no idea where Mara Sov is, and as far as we know, Uldren Sov is still on Mars. If you stand near Executor Hideo, there is a random voice line you can hear from one of the unnamed npcs talking about how he knows his sister is alive at the reef and he wants the guardian to go find her. So, that implies that we evacuated some to the reef during the attack. I'm pretty sure that confirms that Petra is still in control and Uldren hasn't returned, because Uldren hates us. Petra, not so much. Eventually in the d2 storyline, we will likely be returning to the reef. Possibly in Curse of Osiris, but maybe not since Brother Vance has moved to the Lighthouse.