International Statute of Secrecy

The Accidental Animagus fan fiction author White Squirrel speculates that not only did society take an abrupt shift, with the pure-blood mania becoming more prominent than it had ever been, but that the magical society probably started to rewrite its own history, erasing evidence of its dealings with non-magical society, and of its own mutations and shifts over time.

This idea is supported by lesser canonical sources. In England, the Potters argue for the Statute to protect the Muggles,¹ the Malfoys argue against it (to continue exploiting the wealthy among whom Malfoys often marry).² Once it passes, the Malfoys pretend to have been in favour all along.³