tn3270 is only of interest if you want to log into an ibm mainframe
and pretend to be a 3270 terminal.

> I am trying to figure out the main differences between tn3270 and telnet.
> (FreeBSD moved tn3270 from the base to its packages collection last year.)

given how many dinosaurs are left, this probably makes sense.

> I have never logged on to a "real" IBM 3270-class mainframe (just a lot of
> machines that act like it).
> 
> What is a "IBM VM/CMS"?

CMS is a guest operating system which runs within a VM virtual
machine.

VM is an OS for IBM mainframes which runs other OS's, each in their
own virtual machines.

> Is there any current purpose for the tn3270 command?

If you have an IBM mainframe you need to log in to, yes.