Immigration has been uncertain for a long time. YCombinator founders have been working on their startups on 3-month visitor visas. Top graduate students can stay in the US for a year, then they need to enter a lottery to be allowed to stay for another few years. Trump changes barely anything. Unpaywalled: http://archive.is/BHfyN
Trump changes plenty, in the past it was assumed that the USA would honor visa once they were issued, now it turns out not to be the case. Essentially this is breach of contract.
What do you mean by "honor"? A visa don't guarantee entry. It simply gives you a path to enter the country which includes inspection by immigration control. They make that very clear on the USCIS website and all the visa forms.
A U.S. visa in his/her passport gives a foreign citizen permission to apply to enter the United States. A visa by itself doesn’t authorize entry to the U.S. A visa simply indicates that your application has been reviewed by a consular officer at a U.S. Embassy or Consulate, and that the officer determined you’re eligible to travel to a U.S. port-of-entry for a specific purpose.....
At the port-of-entry, a U.S. immigration officer of the Department of Homeland Security (DHS) decides whether to allow you to enter and how long you can stay for any particular visit, as part of the Admission process. Only the U.S. immigration officer has the authority to permit you to enter the United States."
[1]https://travel.state.gov/content/visas/en/general/visa-expir...