Turn off power to Thunderbolt device while leaving data connection
Per the thunderbolt specification, it includes power so if you could turn it off, it wouldn't be thunderbolt. I suppose a device on the receiving end could have a switch or power control to disable draw, but I have not seen any hardware that assumes anything but you get power at the same time / before any data connection comes alive on thunderbolt.
It's an interesting thought experiment to think if we had this to help troubleshoot, but not something I see as practical. You'd need an oscilloscope and trace to determine what's happening in your setup if you can't measure voltage lines with a multimeter.
Some USB devices have external power and won't draw as do some rack mountable thunderbolt audio devices but they never draw power from TB so you might need to look in that direction if you can't isolate it by swapping in and out parts entirely.