One of the ideas going around is that the mystery fruit the Gorosei were talking about is Toki-Toki no Mi (Time-Time fruit). First of all, to dispel a popular idea, that final part of the Gorosei conversation seems almost confirmed to be unrelated to Zunesha. The lookout man who reports the giant shadow is not looking at Zunesha.
The Gorosei did not use Den Den Mushi that time. Also, they presumed that Robin as already been captured. We see that they seemingly don’t have live updates of the situation around Wano or from CP0. They don’t know Zunesha has appeared. They’re talking about something else.
Everyone already knew about Luffy, Kid, Law, Kaido, Big Mom and Robin devil fruits. So why has no one talked about this devil fruit up until now?
“That fruit is only a legend now, even to us”, this is telling us that they did not hear that someone is using or has this fruit since they existed or even before. They only heard of it in stories.
“It is not awakened for centuries upon centuries”, awakened doesn’t mean it’s an awakened devil fruit, but it means that it did not have a user for centuries. The five-elders were talking about the devil fruit, not its abilities or its awakening.
We all know that you can’t hide the name of the devil fruit from the user, “Oda would never make a plot hole like this”. The World government changed the name of the devil fruit only for other people such as in devil fruit books. They changed it so that other people won’t believe such abilities exist.
Toki Toki no Mi is the worst devil fruit that could exist in the eyes of the World government, if someone used it from the void century and travelled to the current time, people who know that this devil fruit exists, they will believe the user.
Toki Toki no Mi is a legend for the five elders, no one used it for 800 years and they changed its name so that even if a user from the void century appeared, no one would believe his stories. Maybe the World government 800 years ago knew that Toki travelled through time, so they changed the name.
The five elders never talked about this up until now, so, they only heard about it recently. We know that there are CP-0 agents in Wano and they heard of Orochi’s speech about how Momonosuke is Oden’s son and he travelled through time.
The confirmation that Momonosuke is alive and that the samurais travelled in time also confirms that the Toki Toki no Mi exists. The World government is afraid that people will find out the truth about the forgotten century, so they tried to erase for centuries by giving it a different name.