@icefox a friend who saw it happen told me that was when the war between the inclusionists and the deletionists ended and the latter won: only notable articles allowed, get your filthy articles about Pokémon and such to Wikia:l.

https://www.gwern.net/In-Defense-Of-Inclusionism was recommended to understand what happened. Gwern there talks about how the Wikipedia people saw what happened and sought to staunch the loss but failed. There was a slight uptick starting in mid-2014 that you can just tell in that plot but… 🤕.