The domestic route that has the most potential of diverting in a foreign country would be anything coming in and out of Puerto Rico and US Virgin Islands. You have the Dominican Republic, Turks & Caicos, Bermuda and Bahamas as all divert options,
Flights from the USVI to the states or PR have to clear customs before boarding. When preclearance is closed, they’re an international arrival. Same with Guam to Hawaii.
When traveling between US mainland to US territories a passport is not required if you never touched a foreign port. It’s suggested you have government issued ID and birth certificate. https://help.cbp.gov/app/answers/detail ... erritories
Traveling to insular territories there is a custom check since Guam, USVI, American Samoa is outside the duty zone of US customs. Still don’t need a US passport.
Puerto Rico is domestic. There is no customs check and flights on all major carriers fly back and forth as if was a state. While we can argue samatics but flights to and from Puerto Rico and to some extend USVI are considered domestic since a passport isn’t required. Now many people will confuse USDA and Customs checks as crossing a border but you never left the United States.
I just did a trip two weeks ago in which I went to St Thomas with my drivers license and connected on a puddle jumper to San Juan. Customs entered my information in the system and asked me If I purchased any alcohol or tabaco. I told him no since I just arrived from the mainland and went thru to board my flight to San Juan. A week later I left San Juan and had my bags put thru an x ray machine for USDA inspection. Used my driver license to pass TSA security and boarded my flight back the mainland no customs, no border patrol, and no passport.