Did you read the brookings essay?

There's no mention of a kill switch. Where are you and the OP coming from?

If it's not, how would they possibly enforce it?

That's an absurd question. Do speed limits on the highway mean there's a kill switch for transportation?

Does having a blade guard on a lawnmower mean there's a kill switch in agriculture?

Does having copywrite length codified in law mean there's a kill switch for books?

There are killswitches in this, in the form of licensure. Speed enough times, and you don't get to travel alone.

Coppright length is somewhat of a red herring here since it's unfair to begin with. No work deserves protection for life plus 75 years.

So what would the mechanism of enforcement be?

Jail time and fines? I don't get what you're talking about