In a post on the official IO Interactive forums (via VG247), the developer has revealed that Hitman: Absolution's "Purist" mode will be available from the get go and will not require a thing to unlock.
Here's the post in its entirety:
It seems voicing your concern helps from time to time. I've just heard back from the team that purist mode is now designed to be unlocked from the get go. now, unless something critical happens, I feel confident that you can all get your butts handed to you from the first playthrough.
For those curious what this Purist mode will contain or omit, this mode will remove Agent 47's "Instinct" feature (same with the Detective vision in Arkham City), and will not even include a Heads-Up Display (HUD). You'll have to rely on your surroundings and be at the mercy of the game's AI with not help whatsoever…just like a real assassin, no?
Will you be playing Absolution's Purist mode or you'll finish the game the usual way first to get the lay of the land? Sadly, we don't have a confirmation if finishing Purist mode will net you an extra Achievement/Trophy.
Hitman: Absolution is set for release on November 20 for the PC, Xbox 360 and PS3.