Even within the 2 hour window.
EDIT: Applies to submissions too.
This appears to be a new undocumented behavior, likely intended to reduce the occurrences of the [deleted] that has broken discussion threads of old.
The problem is that there is nothing preventing users from just editing the comment to nothing, which achieves the same effect, so the addition of the lack-of-functionality makes no sense.
It's true that you can still edit the comment text to nonsense or nothing (as long as you're within the edit window of 2 hours, which hasn't changed). I don't see any way short of strong AI to prevent that, nor should we want to. But this isn't the "same effect". It leaves the shell of the comment intact, including username and time posted, which is a lot more context than there was in the past—metadata, anyone?—and so the threads are less gutted. And, some, hopefully many, users will simply edit their comment to remove what was bad about it.
Edit: I forgot to mention that people are always welcome to email hn@ycombinator.com and ask us to delete specific comments if they're worried about them. We do that all the time, e.g. for people who regret including personal info in an HN comment.