How do i get pop growth now?
Each location consumes food. The more months of food (your pops would consume monthly) you store, the faster your pops will grow. Nearly all other modifiers that directly impact pop growths now dont. You can check it in the PROVINCE food storage.
The absolute number of people you gain is LINEAR to the amount of food your have stored. Building granaries to store more food, means in ingame terms you want to grow faster because you want more peasants in that Place.

But bro, peasants produce more food than they consume in rural, everything will grow endlessly, no?
No, luckily we came up with a solution. And that is Population capacity. I changed pop cap (understood as housing) to be much less dependant on vegetation and inert modifiers. It now scales with the number of STAFFED BUILDING slots you have in the province, meaning if you have a rural settlement and dont invest in it, its pop cap will be much lower than a rural location with tons of farms and laborer buildings, people can live in.

And if pop cap is hit growth just stops?
No pop cap is a very soft system, depending on how many percent you are over your pop cap the more pop growth (and migration for that matter) will slow down. I have seen ai cities hitting 2x pop cap, you are not hard stuck because of it.
Keep in mind i just added this system and will work on these numbers to make them lead to sensible outcomes.

Okay but how do i produce food now?
Food is produced by your peasants, by food rgos and buildings (like farm village, fishing village, forest village). Just check in the LOCATION food production!

What modifies food production positively (% modifier additively)?
Rural provinces, fishing village, farming village, development, irrigation and many other inert modifier (like farmland, coastal etc etc)

What modifies food productivity negatively (% modifier)?
Each level of staffed building in a location reduces food production, winter (especially severe ones), a province filled to the brink with people, getting sieged, getting occupied, raising levies and many many more.

How do i store more food?
Grainaries in cities and many other buildings increase food storage, development does too.

Do you think i should just turn this on mid savegame?
If you dont run other mods i can almost guarantee that it will work, but because of the changes you will likely run into immediate starvation (if you are lategame in vanilla) or into a super population explosion. If you run mods, especially ones that change buildings, there might be bugs.

How does the AI handle all of this?
This is what i test the most and what i put most effort in getting right. The ai now puts much more emphasis on food production to avoid starvation, it also builds much less cities and keeps good food rgos rural.
In total urbanization and roads are GREATLY slowed down for player and AI in this mod. Starvation should and will happen.
The ai will also stop purchasing of food if it would lead to greater debt and rather starve its over urbanized cities to a more handleable size.
This is also the area where patches patches will finetune more and more as it is most difficult to get right to get roughly historical outcomes.

Other changes?
Massive changes to pop food consumption, some changes to buildings and production methods, many changes to ai behaviour to make them prioritize food and much more stuff, you can read the patchnotes and description to get a reasonable full picture

Do you have the mod in a repo?
https://github.com/JanB1989/ProsperOrPerish

Compatibility
Just make a save without the mod activated, turn it on and see if anything is broken, when you run many other mods its impossible for me to estimate if it works. I personally run xorne ai and a bunch of diplo mods, and everything works fine.
Once things have calmed down a bit, i will spend time to make this mod explicitely compatible with some mods. These are probably mods that ahve a host of building/production methods changes and put effort into balancing them. Im sure many diplo mods are already perfectly compatible, i just cant guarantee it.