As I understand it, the Keys are open to anybody, but a lot of things you might want while there are still closed. The motel we use in Marathon, The Blackfin Resort, is closed. All the state parks are closed until further notice. I checked with a couple of charter scuba/snorkel operators and they were closed as well.
I'd check with
http://www.keysrecovery.org/ or google 'Florida Keys Recovery' and you'll get a good over all idea of what is going on at the time you get to Miami.
Our trip to Ft Jefferson and the Dry Tortugas has been cancelled as the National Park is also closed until further notice.