I just tried posting something and deleting it for myself. I don't even see '[deleted]' or anything in the thread. It's just gone completely. The permalink says "there doesn't seem to be anything here" (like it does for empty threads or subreddits). No mention of my username nor the content anywhere that I can find.
Reddit internally saves post deletions but not post edits, so if you really want to delete something you should edit it to some nonsense before you delete it.
Of course, the best way to protect yourself is to never post anything compromising in the first place because there are already a lot of services out there that cache everything, but I think it's a good practice to follow if you're going to delete anyway.
Ah, that's something completely different.
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.
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.
Here's a test I just did:
Also, reddit once said that they only keep the latest version of a comment in their database (not sure if it is still true though).... so if you want to really delete a comment from reddit, you have to edit it to some random text and then delete it.
If it's a link post, then it isn't deleted.