And, theymos emphasized that
even years after the publishing day of posts, if plagiarism found, users will get permanent ban.
I will quote the original post of theymos later, when I saw it again, not found ATM. I don't remember where I read it 
It's written in the topic
Writing a welcome message.
Plagiarism: If you copy some text from somewhere, then you should have a good reason for it, and you must link to the source. Doing otherwise is plagiarism. Changing a few words around doesn't matter. If we find that you plagiarized, then you absolutely will be permanently banned, even if we find it years after you did it.