It's a reference to Eternal September, the name applied to the cultural shift of the internet as the general public started gaining access to it en masse in the 90s.

Sloptember is clearly a reference to this - the similarity being that masses of AI generated content, from social media posts to open source contributions are replacing the human internet. In a way this is related to the "dead internet theory", an idea I previously found hard to believe, but these days could easily be true.

If the history of the internet interests you, both these are worth looking up.