There are a few around, and a few folks ran them for a while to good effect but for one reason or another didn't bother to instal one after a tank reset. But you're right that it's not common at all anymore.
I also wonder if there are more tanks like that than we realize because the reefers who keep them aren't online. When plenums came on the scene, the Internet was a baby. The few reefers who were online were chatting in, like, listservs and aol dialup boards. There's no way to know how many happy oldtimers are chugging along with 20 year old tanks using pennies as a copper source for ich treatment, trickle filters, and Rube Goldberg closed loop systems for flow.
IMO you don't need a plenum to have a thriving reef, and it's not really something to do if you don't have to because of the scale of the project. NB the skimmers back then were crap, and a powerhead that pushed as much as a modern mp10 was about 5" square. So it'd be fun to run a plenum like it'd be fun to build a Commodore 64 to play 8-bit games on.
