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Well, steemit couldn't enforce it. It is all out there for anyone to see.
steemd.com allows you to view all texts of all messages, deleted or not.

You can make a post, and then edit it with a completely different post, and you will probably be the only one who ever goes and looks for it.

You could also encrypt whatever into anything that spits out ascii. Like pgp.
And past that in your post.