It's not new, by any stretch. I remember seeing such many references in the early 2000s and beyond. Doing some vague poking around textfiles.com, I didn't pull any easy to find 1980s 'boxen' references. It almost looks like it might have been the addition of 'unix boxen' to the 'new hacker dictionary' by esr which spread the joke into the mid-90s communities. I wasn't online at that point myself, so I can't speak from any personal experience. But the various hacker zines etc are using 'boxen' by the later 90s and early 2000s, which would allow for it to have spread through the folks of the time finding and trolling through esr's version of the 'hacker dictionary'. https://www.dourish.com/goodies/jargon.html complains about esr adding unix references, and does not include any boxen references that are found in the http://cd.textfiles.com/hackersencyc/ETC/MISC/JARG_331.TXT version folks in the 90s would likely have spread around.
edit: nope. definitely there in the 80s on usenet
so he was just encoding existing slang usage, rather than introducing it, on this point.
https://www.usenetarchives.com/view.php?id=net.wanted&mid=PD...
more edit: considered the second oldest boxen reference in the usenet archive is esr himself, maybe he was the one pushing it :)
https://www.usenetarchives.com/view.php?id=comp.sys.att&mid=...