On March 18, the Government of Canada announced the temporary closure of the Canadian-U.S. border to all non-essential traffic. For more details, see the notice available here.
There are no travel restrictions if someone is flying into the US, except for those individuals meeting the restrictions under the Presidential Proclamation from a few months back on individuals having traveled to/from China, Iran, and certain European countries. If individuals are traveling from Canada to the US for business or pleasure and have not traveled any place that falls under the Proclamation and there are no other concerns of health or admissibility they would be permitted to travel. This is why we have continued to have flights to the US. However, Canadian citizens traveling to the United States will have to return to Canada, even if they have no symptoms, to comply with quarantine requirements and have a place of isolation. The Government of Canada reminds you to avoid all non-essential travel outside the country.
For more information: New measures