After canceling House Speaker Nancy Pelosi’s plane trip to Europe and the Middle East, Trump had acting Office of Management and Budget (OMB) Director Russ Vought send memo to all heads of executive departments and agencies that members of Congress were not allowed to travel on government aircraft without express permission from the White House during the government shutdown.
"In light of the current partial government shutdown, the President has asked me to direct the heads of all executive departments and agencies of the Administration on new policies in support of Legislative Branch travel,” Vought wrote.
“Under no circumstances during a government shutdown will any government owned, rented, leased, or chartered aircraft support any Congressional delegation, without the express written approval of the White House Chief of Staff,” the letter continued. “Nor will any funds appropriated to the Executive Branch be used for any Congressional delegation travel expenses, without his express written approval. Departments and Agencies shall continue to provide appropriate logistical and security support for such delegations.”
Here is the official letter from the OMB: