This is just for you. In my history I see reports that begins in early February.
Counted the number of reports. There are exactly 5000 lines in my post history. Consequently, the restriction occurred not by date, but by the number of lines.
It would be interesting to know from those who have a lot of sent reports in the last 30 days, do they have the same limit of 5000 lines now?
UPD: I sent 50 fresh reports right now and counted the history of reports. The result is the same: exactly 5000 lines. That is, the limit is now on the number of reports in history. It remains to be seen if this is the case for everyone.
Based on the publicly available number of reports handled every month, I don't believe that anyone else is reporting in as high of a volume as you are, nor is anyone reporting in excess of 5000 posts per month. In the year period ending in ~ mid-September 2021, actmyname reported an average of ~3100 posts per month. I am not sure how these reports were distributed over the year, but he is the only one that would even theoretically come close to exceeding your 5000 reports ina month.
I checked my report log, and it goes back 30 days and contains ~1000 reports.