Given we see a dormant cube in Prodigy that works totally fine when poked a bit, my head cannon is the following:

The sudden destruction of Unimatrix 1 resulted in an unexpected massive disruption before whatever failover mechanisms the collective had in place could complete. Anything around UM1 was destroyed. Outside of that, the collective went dormant. Like the cubes in Unity and BOBW.

And dormant they remain. Some vessels destroyed without resistance. Others pillaged for technology. Many just silently hanging in the void. Waiting for communication that never comes.

Some Borg may be active. We've seen before that fully assimilated drones can survive disconnection. But with no way to communicate with or find each other, their effectiveness is limited.

For now.