I doubt there's proof (much like how China harvesting organs from certain races and religions didn't have solid proof till recently), but it's in the right ballpark. 50+ journalists are killed (and documented as such) each year, for their journalism, despite their attempts to maintain privacy. Privacy measures being suddenly dropped wouldn't make those numbers better.

Toss in another 150+ human rights activist murders per year, dozens of whistleblowers, 60+ transgender people, and any number of other at-risk groups who tend to rely on technical measures to preserve their privacy and safety abroad.

Did that info being leaked definitely get them killed? No clue. Did it "probably [get] people killed"? I don't know how those people are typically uncovered, so it's hard to say if a few privacy extensions actually mattered. It doesn't seem like an outlandish claim though (and with a prior that those extensions do matter, it's a likely claim).