iOS has had "Guided Access" since almost forever. Not quite a guest account, but you can use the Accessibility shortcut (from the Control Center or triple-clic

iOS has had "Guided Access" since almost forever. Not quite a guest account, but you can use the Accessibility shortcut (from the Control Center or triple-clicking the sleep button) to quickly prevent app switching before handing a device to someone. You can even use it to disable interaction with parts of the screen by drawing regions, or disable the hardware buttons, and set an autosleep timer. What operating systems really need though, is a "fake OS" mode. It should display an alternative launchpad with some random apps with random photos, notes, messages etc. When someone tries to do something it should stall with poor network connectivity dialogs or other fake issues. Maybe someone can make an app like that? :)