The architecture does indeed come from the late 80s/early 90s, but I think it's more relevant today than ever. Separation of services is, in my opinion, essential to security. By putting the authentication service off in its own machine, you restrict the attacks that can be made on it; the auth server only talks over a highly restricted protocol. On a standalone Unix system, users log in to the same machine that stores the passwords. They're only a privilege escalation exploit away from getting the hashes of everyone's password, and these days privilege escalations are a dime a dozen.

When this scheme was designed, it was frankly a little bit nutty. The CPU, auth, and file servers would be VAX or Sun systems, costing tens of thousands of dollars, and the terminals would be either slightly cheaper Suns or IBM PC-compatibles costing thousands of dollars themselves. Today, you could probably cobble together a passable network for a few hundred dollars, assuming you use cheap Atom boards for everything except the CPU server (which is meant to be a beefy compute box, but let's be honest nothing in Plan 9 uses a lot of cycles). This makes the architecture more sensible than ever.