Illusionism can be defined as the manipulation of in-game probability and description to channel the PCs toward predetermined outcomes.The "Quantum Ogre" (and that series of articles says everything I'm trying to say, but better) -- no matter which patch of forest you explore, the Ogre will be there ... is Illusionism. The minute "the story" is the most important thing in the game, you are well and truly down that path.
Is that a bad thing? I don't know. Consider that game that many of my friends and I regard as our finest endeavour, the Tale of the Tiger Clan, in L5R, was nothing more than an Illusionism-rampant story-telling experience that I ran. Outcomes were, for the most part, completely predetermined. Not really before I ran the campaign, I admit I had no idea where we were going for the first year of play, but certainly session by session. By the time we were into our second year of play the "end game" had pretty much solidified in my head. All I had to do was get them there. And I did.
Despite the Quantum Ogre lurking in either Woods A , B, or C we had fun. Heck, we had a lot of fun.
As I begin to explore Player Agency and how to increase it in my game I look back and wonder how things would have worked out differently. I doubt our epic conclusion would have been as epic, although who knows what other fascinating things might have happened?
In our Kingmaker game we are using a published source. I have added bits, and removed bits, altered things as I saw fit. For the most part the players' characters are free to go where they want in the little sandbox -- but the expectation is still that they'll go everywhere. How much is that sandbox, the thing that got me started on this whole OSR kick in the first place, really just -- an Illusion of Player Agency?