I don't believe we ever wrote it up, it was just an internal algorithm change made in 2016. I can see the internal pull request but don't think we blogged about it.
Seems like an oversight to not promote this change. The way Cloudflare completely crippled the user experience of using Tor, plus the subsequent condescending and poorly handed PR responses I saw on HN and elsewhere, was the reason why I completely stopped using Cloudflare and stopped recommending it to people.
