This is not the sort of thing you typically see in fantasy, though part of it is because readers do not need that level of detail. And, of course, how much detail do you, as the writer, really need to work out for the history?
The answer remains: as much as you really need. But I think it's important to note the difference between simplicity and stability. It's one thing to not go into the details of how a region has shifted hands and borders of the years. It's another to make those lack of details translate into a lack of change.
People don't work that way.
The same could be applied to the "lost king" trope. I can't imagine a civilization would run on the premise that their government is permanently in 'regent' mode, on the hope that the person who really is the ruler might show up and claim their right at any time. That the system is designed with this premise in mind.
I mean, has that ever actually happened in history?
Now, I admit, in the history of Druthal, I do make a little play off the "lost king" trope, in that a group discovers that one of their number is a direct descendent of the first King of Druthal. But they don't use that as proof of divine right, but rather as the spurious grounds to depose a horrible king that they already wanted to depose.* It wasn't destiny, but the sugar they used to coat the bitter pill of revolution they wanted the populace to swallow.
So, it might be valuable to check through your worldbuilding and ask yourself: do I simply not go into detail about the past here, or have I built something that is far too static to be realistic?
*- Druth history is actually filled with bad kings-- either incompetent or malicious-- and people who plot to get rid of them. This is something I used as a general plot point in Way of the Shield.