Watch them find something KKK-related in here.
If they really want to keep their means and methods secret, the way to do that would be by giving their internal complaints system enough teeth to never require leaking full documents in the first place.
Police departments, comparatively, are not that classified or secretive. It's definitely much more likely that there's evidence of undercover operations that were leaked by this, probably. Certainly much more likely than something KKK related, what makes you say that? I admittedly haven't looked to see how far back in time these files go, so something these days seems a little outlandish, at least from what I know right now.
So unless they are very bad at their job at gather intelligence and analyze outcomes, leaks don't cost lives unless they allow it. It like a pyromaniac telling the fire department the exact location (down to the meter) and exact time (down to the second), and what exact mechanism they intend to use to light a fire. The fire department will get annoyed and angry, and the police will likely show up, but the risk for someone to die in a fire is quite low.
Mistakes happen all the time, and that doesn't even get into poor redaction methods that can be reversed, ESPECIALLY since the groups being targeted are most likely able to put pieces together that journalists can't. The mysterious nine character name might be a total mystery to the interns at NYT, but if you're an insurgent with the context of the rest of the report and (most likely) a few good names to guess with it's far from real mystery.
To reiterate, the lives in question are likely still classified, and revealing even their deaths can come at significant cost. Telling terrorists that they have the right guy presents about zero benefit to anyone but them, so there's no reason it'd be public information.
Furthermore, lives are a pretty low bar. I don't think it should be controversial to say that the US government, for instance, should be a few steps ahead of violent extremists. Debating the specifics of how and what measures are appropriate is another very important and necessary conversation, but saying that redacted leaks are totally fine is like giving just the suits of your cards to your opponent. Nothing like this is harmless, that's just myopic.
There is however a lot of conditions for that to happen, which is why the general claim that a leak could endanger lives should be seen as rare, unlikely, while possible event.
To make a guesstimate, journalist and government official risked more life by the additional traveling by plane and car in order to discuss and publically address the leaked documents of Chelsea Manning than the risk exposed by the leaked documents themselves. The agencies involved was likely competent enough to eliminate all higher risks well before the publication date.
> Furthermore, lives are a pretty low bar
I don't think anyone object to that. Leaks should be seen as having a high risk of disrupting operations and increasing resource costs. I would expect that pulling out operatives, protecting collaborators, replacing operatives, and operations that fails are all very costly. The trade between an informed citizens and costs is something which should be more often discussed in politics. Journalists can sometimes reduce the costs with careful work, but it not a clear cut and sometimes they will make a mistake and sometimes its the government that goes to far in hiding too much information from its citizens.