I think 1945 is a decent enough cutoff. You just know the three biggest shitstorms are gonna be centered around the cold war, WW2, and Israel/Palestine. WW2 is too big to be left out, but the other two don't really feel "historic" enough in the first place.
In the grim darkness of Sosach, a slav posted a shot of his cat in front of the Blinis (think of slavic pancakes that aren't sweetened) in which it looks as if it were drooling over the delicious food.
It became an exploitable on Sosach, then on various smaller boards such as Ylilautaa, KC and finally ended up on 4chan via /int/.
I hope you tune in again for the next episode of meme history with Franz-Josef Strauß.
Enough kotblini shitposting.
9/11 is a turning point big enough. It is one of these events to which everything converges, with it spawning all major events coming after it.
Until we have another event that may be referred to as a turning point like 9/11 in retrospect I'd place the cutoff in 2001.