"Caution: This profile may include sensitive content. You're seeing this warning because they tweet sensitive images or language. Do you still want to view it?"
Warren wasn't notified of the change, but he jokingly said that maybe it was because "Americans struggle with swearing as punctuation."
Twitter tells TechCrunch that a profile will be flagged based on your individual settings. Meaning, if you have the box checked that you're cool with seeing potentially offensive material, you likely won't see any of these greyed out profiles. I have that box checked, for instance, and Warren's timeline was totally accessible to me.
We've reached out to Twitter for more information and will update this post should it arrive.
Update: A Twitter spokesperson responded saying that this is a test as part of the company's efforts to make the site feel safer. No further explanation was given for what will lead to a profile being flagged or how widespread the test is, but we were given a few links to peruse regarding sensitive media.