Where do I find the Magic: the Gathering article about varying card backs?
I've found a couple of articles on MtG's card backs, aside from that one in your original post.

One of them is fairly old, from 2002, and it talks about how Arabian Nights was conceived.

The second one is from October 2017, where Wizards of the Coast unveiled their new MtG logo. This one specifically talks about how they have no intention about changing the card backs at any point for the foreseeable future.

I'm still on the hunt for the specific article you mentioned, but I've not yet managed to find it.