Depends on the driver. A light with a boost driver should easily output at max level until the voltage sags enough to cause the driver to step down the brightness.
Your generalized statement only applies to linear or buck drivers (and also depends on lots of variables: the LED Vf at the given output, etc.)
Just did a bench test: S2+ 519A (has low Vf) still draws 4.5A at 3.4V whereas at 4.2V it’s 5.4A, so 83% output.
519A Vf at 4.5A is 3.27V: Nichia 519A - #55 by thefreeman