1) to define the intended rules of the game for the purposes of achievements.
My longstanding gripe with ironman is that EU 4 itself as a poor history of accurately defining the rules of the game :/
When running into issues where a "bad" decision was made primarily or entirely because the game mislead the player, ironman feels like a one-sided covenant. A promise (at least on paper) to not "cheat", even as the game cheats you in a literal sense (gives false information about the consequences of a choice).
That never sat well with me, and I'm glad there's been effort to reduce it. But even with the fixes, it's simply not true that a player with 100 hours or less and I are operating with the same rules in practice. In theory, we're looking at the same game...but I can stackwipe the AI three+ different ways that player doesn't know about (and has no reason to using only the game presentation), in addition to knowing rule interactions that make winning the fight in the first place possible. There are too many things the game does without telling you.
The issue is less so the concept of ironman, and more so its application to a game in the state of EU 4's presentation. "Ironman" works quite well in roguelikes, or even games like StarCraft. But those have no equivalent to trapping yourself by capturing a fort, or to the advantage conferred by knowing how stack swapping influences ZoC to not only allow you to save that same trapped stack, but also do what LOOKS like cheating while running past maintained forts. They similarly have no parallel to subject stealing (without war) or inflating damage per war with occupation transfer.