According to an answer by a Stack Exchange Community Manager:
What can moderators share publicly?
During a suspension, anyone can see that a user has been suspended and
broadly why. There's not a lot of benefit for a moderator to do more
than point inquisitive users to that part of a suspended user's
profile. Since suspended users are unable to tell their side of the
story on meta or chat, the less said the better. Hard to think of a
better way to turn a user bitter than to humiliate them when they are
helpless.
Sometimes, however, the questions about a suspension are . . .
pointed. Secretive systems of justice don't tend to be very fair.
Since users on the site clean up after bad behavior, it's not very
easy for people to even see what the suspended user was doing. It
comes down to trusting either the word of a moderator (with
inscrutable power) or a fellow user. In these cases, my guiding
principles for moderators and CMs are:
Be honest. That might mean telling truths that aren't very flattering of yourself or other people. It might mean highlighting the
mistakes of a moderator or community manager. It might mean
summarizing information that's not publicly available. It might mean
publishing moderator messages and responses. It does not mean
publishing potentially personally-identifying information, which is
never allowed under the moderator agreement. But honesty must always
be balanced with:
Be respectful. I'd say be nice, but that might be misunderstood. Correct misinformation, but don't go out of your way to make people
look bad. Focus on the evidence of what happened and avoid assigning
motives. Assume good faith and take the time needed to remain civil.
Believe it or not, people sometimes respond positively (and rarely
negatively) to this sort of generosity of spirit.
In other words, the purpose of secrecy isn't to hide from public
scrutiny, but to protect users and the site from needless gossip and
drama. If you already have that (and especially if the suspended user
instigated it) there's no real reason to keep mum. Better to have
informed turmoil than misinformed. Meanwhile, don't stoke the fire by
bringing out salacious details that could be left quiet. Nobody said
the job of a moderator is easy.
Why we don't keep public records of suspensions
And in a comment:
Certainly if the user gives permission or is clearly out to mislead
that would be a good time to reveal clarifying details
Why we don't keep public records of suspensions
In circumstances where the suspension is of great concern to the community, & the suspended user has explicitly given permission for their suspension to be discussed, it would seem a little odd to take refuge in "no comment".
