Yes, still relevant. Email clients aren't much smarter than they were previously in terms of HTML, Javascript will always be restricted in email because of security concerns, and unfortunately, you always have to play to the lowest common denominator because you don't know which email clients people are using.
The web, on the other hand is pretty evolved and it's quite safe to assume nearly everyone has a modern browser. Even Microsoft shifted away from IE for quite some time ago.
The only update to that article was that we now make it possible to create animated GIFs from in5, which can be useful in emails.