They wouldn't see the outpouring of support for the brave souls in Russia who take to the streets to protest the madness.
They wouldn't be able to keep contact with friends, family and ex-colleagues who can provide them with unfiltered (or at least with other filters applied) information.
It'll be a possible sanction because it will hurt the Russian population and businesses. But it won't avoid any cyberattack.
And even if you reduced it to a few megabits snuck in and out for the whole country, the military would use that and what's impacted is civilians. Bad for the economy? Meh, if all countries were already convinced to be against russia, then trade would be at a standstill and no comms for civilians would not matter that much financially.
That's not to say it's not worth a try, but I expect it won't be effective, or if it is, I'm skeptical that it would hinder them much more than any traditional trade limits already could.