This happened to me. I was able to notice it from network activity lights and stop it by disconnecting the network. Other people I know weren't so lucky.
I guess current version matters.
They opt you in to it. Possibly repeatedly. But you’re never fully forced.
I realize that’s far from ideal, but as a home user you do have control still.
Staying updated is part of “the Apple way”. If you don’t like it, you’re in for a fight until your hardware loses update support.