It's actually a matter I've given quite a bit of thought to.
Plague's not the way to go - disease in the game is too random and terrain dependent.
Given my proclivity for research (
) I'm a believer that a climatological catastrophe ca. 535 CE caused the historical "Dark Ages" (cf.
Extreme weather events of 535–536.)
If that is what you wish to emulate, then simply have Knowledge and Trade creating Improvements etc. made obsolete by a Tech called "Climatological Catastrophe" or "Dark Ages" or some such.
You can even time it by emulating Civinator's approach as described
HERE.
I'm sorry that I've forgotten the author of this, but you can even "upgrade" to "inferior" units using the following "trick:"
"You can set a Unit to Upgrade to another Unit in the Editor under "Upgrade To".
Then Under "Special Actions" Do Not flag Upgrade Unit. Select a Unit that the CIV can normally build later so when it can be built, the 1st Unit can no longer be built. The only drawback to this is you will have to put up with the Civilopedia stating that the 1st Unit Upgrades to the one you selected...even though it cannot Upgrade."
Governments are a bit trickier: as a rule of thumb, the AI will almost always upgrade to a better "corruption" level gov type. And, sadly, there's no way to make a gov type go obsolete ... Yoda Power once noted that, "War Weariness will make it choose a worse government (for example Monarchy over Republic) if it is badly hit" but that's a tad hard to force ...
... I believe the best you can do is fiddle with the AI Gov choice criteria
as defined by embryodead HERE. Lastly, you can tie production to Government-type dependent Improvements which auto-produce units, "perversely" having a more efficient Gov type only able to produce less-efficient units ...
I think (hope?) that's a start at least.
Cheers,
Oz