My understanding of the 80/20 (20/80) rule is that it is one of those networking myths that some marketer used in a presentation somewhere and it then became embedded in networking folklore - of course I don't have references for this so this maybe just a myth I've created.
I would say that pre-Internet (and remember most "traditional" network design had it's origins solely within the Enterprise) the original idea would have been to design your network/internetwork so that as much traffic as possible was kept local. Enterprise WAN bandwidth was scarce and expensive. And operationally this made sense too - servers and printers, etc, were all on the local network with the WAN used for interbranch communications (internal email say) and perhaps nightly backups to a central archive or some such.
The Internet and the Web changed all that.- now it was possible for customers and the public generally to access the services you made available to them. Also the Internet made bandwidth available for internal communciations via VPN, etc.
So the balance has shifted, but to say it's 80/20 or 60/40 or 50/50 and in which direction, is a guess - each enterprise and system has its own characteristics which means initial network design analysis is more critical then ever - generalisations don't help and in my view should be avoided.
And besides I think you'd be very unlucky to have this a question in any cert exam.
it actually goes all the way back to bridging, and it cam out of local campus design.
WANs to LANs even now have a much bigger difference in performance - very few people (or company offices) have an Internet feed that has 20% of the performance of their local 10/100 or 1000 Mbps LAN.
the idea was that most traffic stays in 1 collision domain and only 20% crosses a bridge to go somewhere else.
this was when bridges (= 2 port switches) were software driven beasties and couldnt cope with a full 10 Mbps thruput.
and even if they could handle "wire speed", with the same speed LANs everywhere and 2 port bridges, if your design has 100% remote traffic over a bridge hop or 2 the aggregate thruput of your campus drops to 10 Mbps or even less.
it went out of the window when multiport bridges and then switches became available.
Plenty of big sites ended up with large central star switches or even routers with just about every LAN hooked up.