This behavior is still common in flashlights. Flashlights using three alkaline (or NiMH) batteries in series, or a single Li-ion cell can drive a white LED via a linear regulator (or occasionally just a transistor), and it will dim as the battery falls below the forward voltage of the LED at its maximum output. At higher price points, a single Li-ion cell and a regulated buck converter is common to see and much more efficient, but maximum brightness is still usually limited by battery voltage.
A flashlight using a single AA or AAA battery must use a boost converter because all white LEDs require about 3 volts. Even these often don't produce stable output as the battery drains, which is sometimes intentional because that behavior would produce terrible battery life with alkalines due to their high internal resistance. It's fine with NiMH.
Even Li-ion lights with boost converters don't always manage full output on a low battery because it's common to find overdriven components on a 20mm driver board (it needs to fit in a pocket) that's trying to push as much power as possible (lumens sell lights). Inability of the electronics to maintain full output isn't necessarily a significant limitation in the real world anyway; a 25x100mm aluminum tube pushing 40W gets hot fast, and there's almost always some sort of thermal-throttling mechanism. That said, full output on a low battery usually earns praise from reviewers.
This behavior is still common in flashlights. Flashlights using three alkaline (or NiMH) batteries in series, or a single Li-ion cell can drive a white LED via a linear regulator (or occasionally just a transistor), and it will dim as the battery falls below the forward voltage of the LED at its maximum output. At higher price points, a single Li-ion cell and a regulated buck converter is common to see and much more efficient, but maximum brightness is still usually limited by battery voltage.
A flashlight using a single AA or AAA battery must use a boost converter because all white LEDs require about 3 volts. Even these often don't produce stable output as the battery drains, which is sometimes intentional because that behavior would produce terrible battery life with alkalines due to their high internal resistance. It's fine with NiMH.
Even Li-ion lights with boost converters don't always manage full output on a low battery because it's common to find overdriven components on a 20mm driver board (it needs to fit in a pocket) that's trying to push as much power as possible (lumens sell lights). Inability of the electronics to maintain full output isn't necessarily a significant limitation in the real world anyway; a 25x100mm aluminum tube pushing 40W gets hot fast, and there's almost always some sort of thermal-throttling mechanism. That said, full output on a low battery usually earns praise from reviewers.