Can you add worldbuilding footnotes in your novel? Most philosophy books have footnotes on them, because sometimes the text by itself is too hard to understand. Are there novels that use footnotes to explain their extremely complex worldbuilding, or it's something no one would do and no one has done? Why is it a bad idea or a good idea?
It is rare yet has been done. I flipped through a few dozen novels and short story collections and found a few in a the book Labyrinths by Jorge Luis Borges
"1 There is an erasure in the manuscript, perhaps the name of the port had been removed." (in the short story The Immortal,)
The bad idea part is it is unusual and could put some people off. The good idea part is it is another tool available and like any tool, if used well it can help craft a masterpiece.
I would guess that the most famous recent example of a novel with footnotes (actually, they're endnotes) is David Foster Wallace's Infinite Jest.
The novel includes 388 endnotes, some of which have footnotes of their own.
But these endnotes are not (only) included because Wallace wanted to share the details of his near-future world unobtrusively. They are a device the author intentionally used to produce a specific effect in the reader.
In an interview with Charlie Rose, Wallace characterized the novel's heavy use of endnotes as a method of disrupting the linearity of the text while maintaining some sense of narrative cohesion.
Wallace explained that the disjointed feeling of flipping back and forth between the main text and the endnotes was intended to simulate the experience of living in the late twentieth century, when competing urgent demands on our attention constantly jerk our focus from one thing to another.
The endnotes are also intended to create a narrative structure that resembles a fractal, or a Sierpiński triangle. You can hear him discuss this idea in his interview with Michael Silverblat: Bookworm - David Foster Wallace: Infinite Jest
Perhaps a more extreme example is Vladimir Nabokov's Pale Fire, which takes the form of a 999-line poem, followed by many pages of endnote commentary. In this case, the entire story is found in the endnotes.
Starting with the epigraph and table of contents, Pale Fire looks like the publication of a 999-line poem in four cantos ("Pale Fire") by the fictional John Shade with a foreword, extensive commentary, and index by his self-appointed editor, Charles Kinbote. Kinbote's commentary takes the form of notes to various numbered lines of the poem. Here and in the rest of his critical apparatus, Kinbote explicates the poem very little. Focusing instead on his own concerns, he divulges what proves to be the plot piece by piece, some of which can be connected by following the many cross-references.
Again, the choice to include endnotes is obvious motivated by much more than a desire to include extra detail, or clarify something confusing in the main text.
The general point I'm trying to make is that using endnotes in novels can be done extremely effectively, but perhaps only when the author's choices are considerate, deliberate, and well-informed (Wallace and Nabokov, like all great writers, are also great readers).
This is done fairly regularly in "hard science fiction" -- as by authors such as Hal Clement or Jerry Pournelle, stories such as Mission of Gravity, Still River, Close to Critical, King David's Spaceship -- and quite recently Outies.
These authors sell their work in part by making it as scientifically plausible as possible (usually giving themselves a pass on one or two items without which there is no story, like a faster than light space drive in a story about the interaction of humans with a non-human intelligent species), and readers like myself often get a kick out of reading the scientific reasoning that went into some fairly bizarre situations or alien species.
This, in fact, is a situation where it may be not just permitted, but highly desirable to include worldbuilding information (from the above authors, often couched as a scientific report or research paper) as an afterword or end notes.
I would say in fiction, if you are creating a new term, it must be explained in such a way to the reader that they can understand it. This can be anything from context clues (In Avatar: The Legend of Korra, the characters learn of something called a "Mover" from Varrick, an eccentric rich business man who supports them. Varrick offers to demonstrate what a Mover is and begins to set up and demonstrate... what is a very early movie projector to a modern audience. From that point on, all the characters refer to film as "Movers" because now the audience knows... in the setting of an early 20th century technological world, the characters not knowing what Movers are is expected, so it's included), from characters inventing the word whole-cloth and describing the function (In Book 1 of Harry Potter, Albus Dumbledore's first acts of magic is to appear from nowhere and use a device to turn off all the lights on the street. The narrator calls this device a "Put-Outer" in Book 1 but in later books, the reader learns that these simple terms describe what the Wizarding World refers to as "Apperating" and "Dilluminators" respectively.).
Or the term is used in a turn of phrase or exchange that the reader can make the substitution easily (In Star Wars, when Princess Leia calls Han a "stuck-up, half-witted, scruffy-looking Nerf Herder" and Han takes offense to "scruffy-looking" to Leia's frustration, the audience can infer that because of the nature of the combination of the three familiar insults (stuck-up, half-witted, and scruffy-looking) and her frustration at where he placed his offense, that Nerf Herding was not a desirable profession or one that was loathsome. We need not know what the hell a Nerf is, nor do we care because of the four insults we do hear, we know three of them, and the least offensive is the one where Han balked. It's akin to someone in the real world calling a person an "poorly dressed, cheap, illiterate whore" and the recipient of the insult taking offense at implication that she does not offer her clients a high quality service for a reasonable price.).
At either rate, your novel should show us what these details are, not footnote it, unless the fictional work is being written in an academic voice (you're writing a fictional text book, where footnotes happen).
After all, in Star Wars, the Clone Wars were discussed as a historical event in the first film, 20+ years before they were ever depicted in a film themselves.