Lol? Really? I did not know that. Why does it work like this? Did anybody try to address a question about this functionality? Maybe it's a bug...
This is an initiative of local moderators. If you send a report to a post before this date, they either put BAD or ignore this report. Well, this is such a local moderators rule. And in the English section, as I understand it, reports can be sent to posts no earlier than January 1, 2017. I once sent reports to 2016 posts , always got BAD.
Each section apparently has its own rules, which are set by the moderators at their discretion.