This has always seemed pretty straightforward to me, so I simply don't store things that need to stay private on other people's machinesBut what's the definition of "other peoples' machines?"
If you run Windows 10, it's not really your machine. You don't have full control over it, do you? It updates and reboots when Microsoft tells it to, not when you tell it to.
If you use a smartphone, that's not your machine, either. If you can't read and write every byte of storage from the baseband processor to the video chipset, but the company who sold you the phone can, then that means it's their machine, and not yours.
If you use a Linux box or a Mac, I hope you never plug an Ethernet cable into it, because that effectively turns your machine into a dumb terminal on a system that isn't yours, just as the Oracle (Corporation) foretold.
"Keeping it to yourself" gets a little less practical every year, it seems.