Ironman modes for games generally are when saving/loading is restricted to exiting the game, or not being able to save at all. Basically, if something goes wrong, too bad, deal with it, and zero reloading for any reason whatsoever.
Champion glitched and disappeared? Sucks to be you (but it happens to the AI too occasionally). Misclicked in diplomacy and declared war? Well, I hope you're on war footing. Got really unlucky in a battle and lost? No retrying
There's no actual setting for it in the game, but that's why it's "psuedo-ironman"; you just have to keep yourself from reloading an earlier state of the game, which also tends to help with stability (or so I've found).