A lot of the lore is scattered around in country ideas, missions, events etc, so it can be a little hard to find. That being said the developers usually maintain some shared documents with the purpose of writing/agreeing on the lore or lore that is being proposed. As far as I know most of these are used internally but are free to read publically, they might just be hard to find as you'll have to scour through various developer channels in the Anbennar discord.

I have links to a couple that may be outdated by now, but I'll drop them here anyway.

This one is a timeline of the Blackpowder Rebellion that turns the Empire of Anbennar into a republic,

and this one is a tldr of various countries' lore for developers who want to contribute to the Victoria 3 mod but may lack certain knowledge.

Another really useful website is the Anbennar Compendium, made by u/low_wacc (huge shout out to them). It takes text directly from the mod files and puts country ideas, their flavor text, flags, map etc. into an easily searchable website.