Is it done for the sake of transparency or is it a software limitation? If it's the former, then it's just clutter and almost as harmful to the health of a thread as public ban messages on 4chan-based imageboards. It isn't like you know what, when and why it was deleted, only that something used to be there.
Just my two cents, anyway.
Huh, it actually says あぼーん on /nihongo/.