DC dimming LEDs is relatively easy, and somewhat common. The problem is that it's expensive compared to PWM dimming. It requires more expensive current-adjustable circuitry.
Additionally, for bulbs that are used in regular household fixtures, they basically need a way to convert TRIAC chopped 50/60Hz AC into constant current... which makes things even more expensive. Smart bulbs that are supplied a constant non-chopped AC can do it easier, but it's still expensive to do DC dimming.
I guess there is some threshold below which the LED turns off so the voltage/current -> light function needs to be set accordingly.
When I was in high school we were messing around with liquid nitrogen and overvolting LEDs and noticed the odd effect that the color of the LED would change if you overvolt it. It was years before I found out why
Voltage, yes. Current, no not really. You can drive extremely low currents and still get photon emissions from LEDs. That said, it's highly non-linear, so you basically need to assign set points. Doubling the current won't double the lumen output.
Additionally, for bulbs that are used in regular household fixtures, they basically need a way to convert TRIAC chopped 50/60Hz AC into constant current... which makes things even more expensive. Smart bulbs that are supplied a constant non-chopped AC can do it easier, but it's still expensive to do DC dimming.