I see in the patch notes that it was being given prematurely, or something. But I've already done it.
A week or so ago, I was flying to DS9, as normal, when I got a distress call. It was being invaded by the True Way! I went there, completed various space and ground portions of the mission, and completed the whole thing. It was actually really really fun. A few days later, I look at my contacts, and see that Yanishev has a mission for me that sounds awfully similar to the one I did already.
It seems to me that the mission is ready to be played, because I played it, with no bugs or errors or what have you, and it was a blast. I guess they need to make it into a fleet action, rather than a solo mission? Not sure what I'm trying to say, except wonder what the heck is going on.
There are at least 2 other missions I can think of that take place in and around DS9 (though the names escape me). The one in the patch notes referred to a Fleet Action (thus it being given by Yanishev) that has not yet been implemented.
A lot of people are falsely saying the Fleet action is not implemented yet. It is implanted; lots of people have ran it. I went in twice and it was full of people.
False. The "DS9 is under siege!" mission refers to the same fleet action confirmed by Cryptic to be not yet implemented. There are no fleet actions currently in game at DS9. I assume you're referring to the earlier True Way attack on DS9 mission. The space parts of that mission contain numerous starfleet npc ships that I assume you are mistaking for player ships.
I'll have to check my captain's log later, but the mission I ran fit the description of the mission being given by Yanishev very closely, if not exactly. It was most definitely a single player mission, but maybe that's why they're holding it back: to change it to a fleet action?