This mod has not been forgotten. I have already merged it and made it 100% modular. It will be included in AND beta7. It's a custom game option.
I made a few tweaks. I left the calculations the way dexy had them, but I changed the effects of a dark age. They are as follows:
If you a plot producing more than 2 commerce or production, it loses 1. If you have a plot producing more than 5 food, it loses one.
Cities grow 25% slower in dark ages
GPP accumulate slower in Dark Ages
If you are using the Revolutions Mod:
There is a one time 250 Revolution Index Hit for all cities starting a Dark Age
A negative 10 revolution index for all cities during a dark age