What is the 20-year Rule? I'm assuming it is some sort of rule about not teaching events less than 20 years old, but I am a teacher and am aware of no such rule, formal or otherwise.

As others have said, there is no real life 20 year rule, it’s purely a convention on the sub.

That being said, you might be interested in the links in this thread about how WWII and the Holocaust were taught in East and West Germany after the war: https://www.reddit.com/r/AskHistorians/comments/81pg6b/im_a_west_german_high_school_student_in_the_1950s/