I think it stems from GG originally, because plenty of the journos, activists, and pundits involved had very low follower counts, and Reddit tried to use that to stop criticism. KIA1 pushed them to give a firm number, and I think that was the number we finally got from them.

500 is low enough that I feel like someone who truly isn't relevant can't be targeted by a board, but also: we can call out people who are significant in specific niches if they are above 500 but below 10,000.