Yes, “headcanons” (one word) is a term. Has been for awhile.
Interesting conundrum: while the idea of an agreed-upon “canon” has been treated with increasing skepticism in literary studies (though not by all parties), the concept thrives in popular culture, especially fan culture. Though its meaning slightly changes. Here’s the definition of canon and a corollary, “headcanon,” from Urban Dictionary: “Used by followers of various media of entertainment, such as television shows, movies, books, etc. to note a particular belief which has not been used in the universe of whatever program or story they follow, but seems to make sense to that particular individual, and as such is adopted as a sort of ‘personal canon.’ Headcanon may be upgraded to canon if it is incorporated into the program or story’s universe.”
So pop culture acknowledges how subjective a “canon” can be, but it also yearns for the authority the word “canon” once had, and grants that authority to show-runners and story-lines when their show’s details (especially plot developments) appear explicitly to confirm a particular interpretation that was circulating among the fanbase. In some cases, particular fanbase theories or “headcanons” actually influence creators working on new episodes. Happened with Charles Dickens’ serial novels in the 19th century, and happens now.
What seems to have been jettisoned is the old notion of a key set of works in a particular genre that represent the best and the most influential works in that field, ‘canonical’ models for others to copy and try to surpass or challenge. But has it? Pop cultural history hardly does away with the idea of “canonical” works you need to know well if you’re going to have any claim to legitimacy in that cultural field as a fan or creator (or both). Its fans are fanatical (and can argue fanatically) about what’s the “best” in a given terrain or network vs. what’s more secondary or tertiary. And pop culture also repeatedly proves T. S. Eliot’s famous notion that a strong new work rearranges the past (the canon), elevating the status or importance of some of its predecessors or antecedents and consigning some other works suddenly to seeming rather old-fashioned or out of it (which is one reason for why works slip out of the “canon”).
Some canon-drums for you to think about today…