Star Wars: The Last Jedi did a lot of things that got a lot of fans talking. There are numerous aspects of the film that are being dissected by fans, but director Rian Johnson just weighed in on one of them himself. At the end of the film (are we still doing spoiler warnings?) we learn that the Luke Skywalker that we've been seeing on the surface of Crait isn't actually there, but instead a sort of projection created by Luke on Ahch-To. Recently on Twitter, a debate started up regarding whether or not this really worked in the film, considering the ability to do such a thing had never been previously hinted at in the Star Wars franchise. Rian Johnson ended up being tagged in the conversation, and Johnson did end up chiming in on the topic, where he pointed out that really none of the Force abilities are ever specifically called out prior to being seen on screen. According to Johnson...
I should stay out of this but... were ANY aspects of force use in the OT (force grabbing objects, force ghosts, even the jedi mind trick) explicitly "set up" before they happened? I'll take my answer off air.
It's a fair point. In the original Star Wars: A New Hope, Obi-Wan only speaks about the Force in vague terms, it's an energy field that surrounds us and penetrates us, and later says that it can both obey your commands and control your actions. In between we see him use the Jedi mind trick and make a Stormtrooper hear something that's not there, before pushing Luke to "use the Force" and go all Bend it Like Beckham on a pair of photon torpedos.
We see the Force perform acts of telekinesis but it also allows for mental manipulation. The idea that you can pull an object to your hand probably makes sense once you learn you can use the Force to strangle somebody, but there was no reason to believe you could use the Force to strangle somebody until we saw Darth Vader do it. We didn't know the Force let you shoot lightning out of your fingertips until we saw the Emperor do it.
There's an argument to be made that Luke's projection is just a major enhancement of a previously seen Force ability. If a Jedi can make somebody hear something that isn't there, as Obi-Wan does while shutting down the tractor beam in the first film, can a Jedi also make somebody see something that isn't there? And if they can, could that be how Luke is appearing on Crait, a mass Jedi mind trick? This would certainly be taking the idea to an extreme, but we're certainly led to believe that what Luke does is extreme.
Odds are Rian Johnson's explanation either wasn't necessary or it wasn't enough. People's opinions of Star Wars: The Last Jedi seem to be held pretty firm and they're unlikely to be changed.