You see this in lots of long-play shows/properties. Adventure Time comes to mind. A number of seemingly irrelevant details become central to the main plot in much later episodes (years/seasons later).

I am wondering, however, whether there is some sort of plot bible that lives with the “keepers” of the storyline, or whether some details are just randomly sprinkled here and there as hooks with the hope that writers will weave them into the future plot.

Either way, this type of writing is extremely rewarding to long-time fans of a show.

Doctor Who is similar, with plot twists often spanning seasons.

"And you, daughter of London, there is something on your back." -- Lucius, the chief auger of Pompeii

That quote raises goosebumps on my arm every time I hear it...

Archer is also known for this sort of thing.

https://www.engadget.com/2015-03-21-massive-archer-easter-eg...

That is a surprising thing to note. Though it does help set up two of the most important scenes in the movie: Lightsabers are used in the dramatic battle between Vader and Obi-Wan; It's good to have them in a few scenes if the first time they're in a battle, the mentor is killed. Furthermore, at the end when Luke is doing the trench run, he's relying on his targeting computer until force ghost Obi-Wan tells him to use the force. This relaxing and trusting in the force was directly set up in the Falcon training scene.

I think overall this is the best way to use Chekhov's Gun: Introduce several things, but don't make it obvious how they're going to fit together.

Even though Luke never uses his saber again, its introduction and exposition gives some heft to the later fight between Kenobi and Vader.

Wow I never realized that. Did they already intend to film a sequel and call back to his lightsaber, or was that just a random detail they worked in later?

Lucas definitely had sequels in mind, but it was very much in doubt whether the movie would succeed. There was an official sequel novel by Alan Dean Foster called Splinter of the Mind's Eye released in '78, which was explicitly designed to be filmable on a very low budget if the original didn't do too well. That does feature Luke duelling Vader with his saber.

It also (rather uncomfortably in light of later revelations) features Luke & Leia as a romantic couple! Apparently Lucas signed off on Foster's manuscript, so he hadn't had the idea that they were siblings yet.

Luke and Leia were teased as a romantic couple in Empire, she kissed him on the lips to draw Han's ire, and then it's hinted that she's his sister in the final scene of Dagobah/when he calls out to her via the force. The incest thing is something that Lucas was deliberately playing with, whether Splinter took that into account or not.

How do you get “deliberately” from that?

The comment you’re replying to is correct; there’s plenty of evidence that Lucas made up both the plots and crucial details of all the Star Wars films as he went along, rather than plotting it all out in advance. Telling people he had, or at least allowing them to believe so, was just another inspired marketing trick.

He has blinders on and is learning to use the force, which he does use again. his lightsaber isn't used again, but they are used again in a master's fight.