Zonk can you tell those of us who don't know this mod how it works ?
Essentially - it was kind of like corruption + unhappiness baked into single mechanic.
Cities would start inching towards revolt if they were far from the capital without connections to it, cities would start getting worried if your treasury was nearing zero and your per turn balance was negative, cities would inch towards revolt if they were just plain unhappy. Multiple religions was a big revolt issue. Certain civics would contribute to revolts, sometimes with different combinations of buildings or religions.... For example - you never wanted to run a theocratic state if you have cities with multiple religions.
Eventually, cities would start making demands -- you could pay off the revolters, draining more gold - but at the same time, increasing the likelihood of other revolts. Sometimes they would demand certain things - buildings, or luxuries. Sometimes they would demand new leadership (i.e., you'd have to give up control -- if you played as Washington, they'd demand Lincoln - and if you accepted, Lincoln AI would control your civ for a certain number of turns).
Or - you could tell the revolters to shut up and get back to work... and if you didn't manage the revolts -- 'separatists' would begin springing up.... armies -- sometimes under your own flag, but using the expanded Rise of Mankind Civilizations -- it was usually a 'natural' revolt (i.e., playing as England - the revolters might be American, Indian, Scottish, Australian, etc).
Sometimes they'd be powerful enough to take a city or cities outright - and break off into a new civilization.... or - even if your repulsed the initial revolt, reinforcements would arrive.... sometimes with a settler, and even if they couldn't pull off a city capture - they'd found a new civilization, usually smack dab near your border, always hating your guts.
Revolutions could get out of control very easily - and it would put a hard stop on ICS until you built up your infrastructure to support additional cities.
Revolts could be empire-wide -- if you had neglected infrastructure - you'd be facing stacks of separatists all over, or, they could be localized (if you settled on the other side of the continent -- even if your empire was largely satiated -- the isolated city could still develop revolts and try to declare independence).
The same mechanic also limited conquests -- conquered cities would naturally revolt, but not always just to rejoin the civ they originally belonged to.... often, they'd break off to another offshoot of the original civ, sometimes, but not always friendly to the original owner, etc.
ICS was nerfed by simply making it impossible to expand beyond your means -- either you were forced to stop expanding because you needed all your forces to deal with revolters, you had to pause your expansion plans to provide what the revolters wanted, or, you had to slow the sprawl and focus on infrastructure to stabilize the empire before beginning another expansion push.
EDIT: The mod is
here -- it's also baked directly into A New Dawn.