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Its atleast disabled by default in latest versions of Ubuntu and other distros, but regardless paul_f shouldn't have posted it without any disclaimer.

It'd teach them not to run random commands they found on the internet, what rm does, and to make backups. Three valueable lessons.
That is, if it would actually work: modern rm implementations have a special case for /. Read the manpage and/or try this instead:
rm -rf --no-preserve-root /

If you blindly run code you find on the Internet without understanding what it does, you deserve all you get.