Nothing will happen. I repeat, nothing will happen...
This is the usual blow hard smoke screen tactics we've seen for years.
I'm confident nothing will happen because "the internet" doesn't care about the powers that be political desires or aspirations. So Trump thinks he can roll back the "Communications Decency Act" sure, if some how the majority of congress acted today (which will never happen) and ratified whatever changes Trump wants AND it isn't mired in legal challenges for years if not decades, it will take all of 12hrs to spin down any US based servers and spin them back up in Ireland. There is zero chance twitter plays ball because they don't have to. All of the big tech companies (some I have worked for personally) don't need nor care about borders, and they can easily be circumvented. The US is 10+ years behind the curve in terms of what China's got going on, there is no way that I am aware of where a US government agency could police internet traffic at the perimeter.
I tell all of my new technicians, "once you know the rules of the game, you can bend those rules to your will." Lets look at R2R, if Trump decided he hated saltwater aquaria and wanted to shut down R2R (my hope revhtree and the other have a good IT team) but if it were my project to run, we would effectively export the virtual disk that houses a good, recent database copy, rent out a VPS somewhere else in the world, spin up that exported system, make some DNS changes and be back in a few hours. It would be 10x easier if we were staying on the same platform. Take AWS or example, its trivial to change your point of presence from US West to UK Ireland, or almost any other country in the world.
If you have ever done anything with network security you know how much of a cat and mouse game it is, myself and two other dudes could easily stay ahead of the US government. Want an example, just look to the pirate bay, MEGA(upload) or any of the other services that governments have tried to take offline.