How common is MAN?
It's just a term. One that they thought would be common, but isn't. Really no one used it and it's just silly. LAN and WAN still describe all of the real types that matter. The idea of a MAN is kind of meaningless. So while they kind of exist regularly, no one talks about them.
It is also implied that it is something managed by the ISP (aka telcos back when it was a term actually used).
Used to imply, yeah. Today it is often down by owned wireless. But the first MAN I worked on was one we built privately in DC. No telecoms involved. That was early 2000.
First one I worked with was in some Citibank offices in the mid-late 90's in St. Louis. Telecoms involved only as far as providing the clean pairs of copper. Otherwise, Citi did it all themselves also.
I was similar. AT&T provided the fiber in case, but we managed everything else.