December 17th 2018, maybe? I know I stopped seeing a lot of tumblr links when that policy changed, even ones that weren't affected by the change. I'm curious how much traffic and posting changed when that finally went into effect.
https://trends.google.com/trends/explore?date=today%205-y&ge...
I will say though that the "final straw" dropped interest by half overnight.... so it certainly was detrimental but they certainly where losing interest over time.
App store bans and country bans should be better indicators.