The tiny ring of invisibility which Bilbo uses as a plot devise to escape a few times, turns out in Lord of the Rings to be the most important artifact in Middle Earth.
About as Chekhov's Gun as it gets.
I wouldn't say that leaving frequent references to the depth and age of the world, and then filling in the Legendaria in note form for the rest of your life, is much related to this concept. If a sword hanging on the wall belonged to an ancient hero, already dead, and known to everyone in the scene, a paragraph with "Here is hung Such-and-such, the bright blade of so and so with which he did $mighty-deed" doesn't have to carry any more weight in the story. There might be a whole book or chapter about so and so, there might not be.
The tiny ring is not the only detail in that book. Checkov gun is that everything must have purpose, not that a thing with purpose exists somewhere inside.
About as Chekhov's Gun as it gets.
I wouldn't say that leaving frequent references to the depth and age of the world, and then filling in the Legendaria in note form for the rest of your life, is much related to this concept. If a sword hanging on the wall belonged to an ancient hero, already dead, and known to everyone in the scene, a paragraph with "Here is hung Such-and-such, the bright blade of so and so with which he did $mighty-deed" doesn't have to carry any more weight in the story. There might be a whole book or chapter about so and so, there might not be.