On Dec. 4th, the Supreme Court issued a decision that allows the U.S. government to immediately enforce the President’s Sept. 24, 2017 Proclamation restricting visa issuance from eight countries: Chad, Iran, Libya, North Korea, Somalia, Syria, Venezuela and Yemen.

On Dec. 8th, the U.S. Department of State (DOS) issued guidance regarding the immediate implementation of travel restrictions under the ban.  Nationals of the eight countries can expect that DOS will not cancel visa application appointments abroad; however, DOS will deny visas unless an applicant is exempt from the ban or qualifies for a waiver based on having “close family” who are U.S. citizens or lawful permanent residents.

Visas issued before the ban became effective (Oct. 18, 2017) will not be revoked.  The ban does not restrict the travel of dual nationals, so long as they are traveling on the passport of a non-designated country. Additionally, this ban does not affect lawful permanent residents.
The DOS guidance can be found here https://travel.state.gov/content/travel/en/News/visas-news/new_court_orders_on_presidential_proclamation.html