The original iPhone UI had “slide to unlock” written on the lock screen slider, because it’s not obvious until you learn it. On the emergency/power-off screen, there’s still “slide to power off” written on the slider. Recently I tried to explain to my father remotely how to restart his iPad. However, in his local language, there is only the equivalent of “power off” written on the slider (possibly due to space constraints, as the language is more verbose than English), and so he was at a loss of what to do, since he didn’t recognize that it was a slider.

Looking at the screenshot of the SOS slider, I would have a hard time recognizing it as a slider myself, when seeing it for the first time.