EzekielRaiden
Follower of the Way
I mean, most canned adventure paths are either linear or very close to it, and I can absolutely say that I have experienced (and been told of such that others have experienced) adventures that either were straight-up linear, or were so close as to not meaningfully matter.I'm only describing using your definitions what others mean by 'linear adventure'. I think @Scott Christian nailed it with his story board picture he posted a few pages back.
I'd suggest that linear adventures don't actually exist (as in no one plays one) with your very narrow definition here. I'm not sure why you've defining 'linear adventure' so narrowly that no one can be said to actually play one.
Illusionism is often used to disguise linearity, for example. The players are presented with something that seems to be a choice or an open-ended question, but in reality there is only one outcome. The fictitious selection (in the trivial example, "road forks, but DM knows that the haunted mansion will be on whichever path the players take") gives the appearance of nonlinearity, without having to do any of the work involved in actually making a nonlinear adventure. Dragonlance was somewhat notorious for being particularly linear as I recall. Similarly, many computer games have truly, completely linear stories. Some of them are quite good stories, but they are linear nonetheless.
In a thread about illusionism from several months back, a poster very explicitly said that becoming a DM means losing the ability to pretend that your choices actually matter. You've seen behind the illusion and can no longer go back to unquestioning belief in your freedom to direct the game, because the DM is (they seemed to be arguing) inherently an illusionist. Pulling the players along through their pre-written story, merely using stagecraft and theatrics to make players believe they're actually making choices. Obviously I don't share their views, but you couldn't have such a poster if no adventures were ever linear.