Comments that are deleted (either self-deleted, removed by a mod/admin, or deleted from account deletion) will disappear as long as no one replied to it. This behavior is the same, no matter how the comment gets deleted.
If the comment has at least one reply, then the username will become [deleted] (or [removed]), but the comment/content will remain.
>If the comment has at least one reply, then the username will become [deleted] (or [removed]), but the comment/content will remain.
That's false. Both the content and the username will be replaced by [deleted] if the user (or a mod) asked for the comment to be deleted, and it had at least 1 reply.
Ah, that's something completely different.