The Wormhole wrote:
The destruction of the transwarp conduit network in "Endgame." It rendered the Collective unable to use transwarp drive and represented the greatest setback they've had from any species apart from 8472. As such, they came to view the Federation as an existential threat requiring immediate extermination.
As I recall, they only destroyed that one Transwarp Hub that was in the nebula. But Seven of Nine noted the Borg had a few others kicking around.
"Endgame" is fairly ambiguous, actually, but I certainly walked away from that episode with the impression that the entire network of transwarp apertures and their hubs had been destroyed, and that's the interpretation that the Destiny
trilogy is built on.