User blog comment:Rappy 4187/Technical Update: September 7, 2015/@comment-3388044-20150909041557/@comment-126761-20150911121559

We identified the source of this - recently we had to make a performance tweak to prevent some queries from timing out, but it went too far and had this unintended effect on very busy communities. We've relaxed the rules, which should mean the histories can get longer again, and we're discussing how to avoid this kind of situation again in the future.