Well it raises a fair question.
The rule was added somewhere between the 06/05/2016 and the 21/11/2016 (I haven’t seen any other historical snapshots in between to narrow it down any further). It’s over two years, but the rules are the official unofficial rules, and not a set of common law articles which normally are not retroactive. By that, I guess that cases are really down to admin/mod best judgment, and they may or may not consider the date as a factor when presented with a case of plagiarism that occurred before mid-2016.