I don’t really know how a dead man switch would work in the cloud, I think you would always need a hardware component for it to work. There’s a few options available like BusKill the can shut down your computer if the cord gets disconnected. You also have options like Presence Sensing on Windows that can automatically lock your machine when it detects you leave if your computer has the appropriate hardware, I don’t think you can set it to fully shut your machine down though.