The recent shift in U.S. policy toward NATO has led the Portuguese Defense Minister to deem a potential F-35 procurement unlikely. In an interview with
Trump’s controversial remarks about NATO, questioning member contributions and even suggesting the U.S. might not uphold its defense commitments, as well as the decision to halt the deliveries of weapons as well as the sharing of intelligence with Ukraine following the meeting with Zelensky at the White House, have raised alarms across Europe.
TL;DR: There is no kill switch per se (as in flip it and brick the thing), but a huge dependency on logistics, services and systems that operators outside the US have no influence over. That in turn will cause the plane - while still flyable - to be grounded over time as it cannot be maintained, rearmed and operated effectively anymore.