I think you deliberately grouped those two words together when that was not the author's intent. "collectively stopped planning" and "stopped planning collectively" have different meanings.
Humans do make plans for the future, especially humans who have agriculture. I agree we don't come together like a big hive mind and dictate what should happen next tuesday, and I can't see that could work at any level.
Technically though, we do form joint intentions in pursuit of long term goals. Decisions like "if we build the bridge from concrete it will cost twice as much but will last for 200 years vs until the next flood" need to be made by communities either via the fiat of a successful / popular overlord or jointly via consensus protocols. At various times there are different rational horizons for planning and coalition sizes. Possibly in times of greater change timer horizons are shorter and coalitions smaller? Hello intelligent agent community ! I'm asking you!