Most liion cells we use are more linear than that.
For example Samsung 30Q 18650:
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You can get power from the cell all the way down to below 3V, but 3V LEDs run at ~3.3V at full power, so single cell flashlights will start to lose the high modes or fade in their output around there, unless the flashlight has a boost driver and a 6V or greater LED, or if it’s got a buck driver and 2s cell config, etc.