Rogues can still do that, they just have to rely on distractions and trickery more. Don't forget there are utility powers that can help with this.
Oh, right. There's, um..
..um..
Okay, let me go over the utility powers here. Just the ones with stealth applications, this could take a while otherwise.
Fleeting Ghost (at-will, 2): What this does depends on who you ask. By an absolutely literal reading, this does in fact let you hide without any cover or concealment, which is pretty fantastic if so. On the other hand, it seems like the intent is to prevent the movement penalties to stealth rather than just be a blank check. Needs clarification.
Master of Deceit (encounter, 2): Useful for starting a distraction, but the problem remains that distractions are 1/encounter.
Chameleon (at-will, 6): Actually if you read this literally it's the best stealth power in the game. Reading literally, you can move out of concealment, trigger Chameleon, and from that point forward remain hidden while whaling on him with a baseball bat as long as you're concealed again by the end of your next turn. This should be the power swap of choice for ninja warlocks in that case.
Shadow Stride (at-will, 10): This.. actually appears to do what Fleeting Ghost is apparently supposed to do. At best it's identical to FG and at worst it's much worse. Weird.
Hide In Plain Sight (at-will, 16): This is a great ability, don't get me wrong, but you have to already be hidden to use it. So it doesn't help with the newly-introduced problem.
Hide From The Light (daily, 22): This is nice but, again, requires you to already be hidden. This being level 22 and daily indicates to me that Chameleon isn't supposed to do what it's written as doing.
Impossible To Catch (encounter, Master Infiltrator PP): Oh! This is the one you were talking about, right? The utility power that now lets you hide again without having to run away, around a corner, and back out from around the corner. I guess that works, yeah.
There are a bunch of utility powers that let you
stay hidden, but that's not the problem anymore. The problem now is that you need to run off and around a corner before you can
become hidden. There is one power that unquestionably addresses that (once per encounter), and it's actually a paragon path power. Fleeting Ghost addresses it with an extremely literal interpretation; if you are interpreting very literally then it's true that Fleeting Ghost and Chameleon are all you need to be as the ninja.