Interesting question. Feels like they could maybe do this effectively for a significant portion of the population but that more savvy users would find ways around a blockade?
It would help to live near the northern border - one could get cell signal from "the other side" in certain situations and effectively restore a limited connection to a limited area.
https://www.accessnow.org/issue/internet-shutdowns/
https://www.accessnow.org/keepiton-faq/
Related news story:
>Internet shutdowns: The new authoritarian weapon of choice
https://www.protocol.com/policy/internet-shutdowns-report (Sep 2021)