I am amazed by the amount of people misunderstanding the notion of a separate Ironman mode in a multiplayer game.
Yes, you can simulate Ironman by yourself by deleting your save, but the point is to have a group of people playing together, agreeing to a set of rules, and all of them being forced to follow the same set of rules. "I've killed you, now delete your save!" isn't something you can enforce on a fellow private group member unless the game gives you the means or the tools to do so.
And before you all whiny about playerbase separation, think a little bit about all the private player groups and solo players out there. Wake up and smell the roses, the playerbase has been separated for years already.
Oh well... there isn't any money in it, so either way FDEV won't do it.