Frankly speaking, I have grown to despise the term "filler" as it became more and more proliferated throughout fictional fandoms. It started in the Western/Anglo Anime-sphere to refer to episodes and stories that weren't direct adaptations to a manga. It then quickly morphed into a pejorative to refer to Anime content a person dislikes/feels it isn't worth viewing. (I have so many issues with this habit alone.) Then it morphed beyond the Anglo Anime-sphere to any fictional works and media to denote aspects and facets deemed "unimportant", "worthy of dismissal and disregard" and other negative veiled and not-so-veiled hatreds and prejudices.
The term "filler" has really messed up fictional analysis and discourse, because you can't talk about something without someone else chiming in "but it's filler; it doesn't matter/count", even though it did happen and therefore did matter and does count. "Canon vs Non-canon" is another analysis and discourse hindering terminology.