It broke, then eventually got delisted. Presumably the person working on it left and no one cares to work on something that clearly won’t lead to promotions? There’s a “Personal Blocklist (not by Google)” now.
Google used it to detect the SEO spammers, improve the algorithm for a while, and now it seems that spammers have found a way around that again because in the mean time maintaining it wasn't worth it inside Google.
