Canada

No credible expert believes that the US has any kind of "kill switch" in the F-35, for the record. Such a system would be almost impossible to completely conceal from the engineers who would have to maintain these planes in service and the risk of that being discovered and instantly tanking the entire project would far exceed any benefit. Remember, the point of the F-35 was to arm the whole of NATO with a single attack fighter. The US benefited plenty from the project as it was, they didn't need to install kill switches, and back under Bush and Obama there was zero motivation for them to do so. People forget how long projects like the F-35 take. They didn't just start building this thing yesterday. The plane first entered production in 2006, and that was after a lengthy design and development phase stretching back well into the nineties.

The concern is not that there might be a "kill switch", but that the US insists on controlling the supply of firmware updates, which would represent a serious risk in its own right, not in a "planes falling out of the sky" way but definitely in a "We can continue to upgrade our planes while locking you out of upgrades" way. It's the sort of thing that, if applied over a decade, could create a serious capability gap between the US and anyone else with the F-35.

NB: To clear up another point of confusion, it is very specifically the firmware that the US controls. Everyone can make their own parts, but they have to load US firmware onto those parts. This another reason why it would be basically impossible to conceal a kill-switch; everyone has the full technical package, they know what's in this thing. Even a tripwire hidden in the firmware would still need some means to be remotely activated, which would be very obvious. This is a stealth plane, all forms of communication in and out are very, very tightly controlled. You can't just slap an extra radio in there.