Cateye did a good one, 2 rows of 5 LED's that can be set idependently i.e one flashing and one constant
A better option is just to have two rear lights. One permanent and one flashing. You can never predict when one may fail with flat battery, especially when riding at night, looking in the forwards direction, with the rear lights out of sight.
Mode # 7 of the Lezyne Strip Drive 300+ would cover that. It's a good light, but not cheap.
Garmin Varias have a mode which is always on but pulses from bright to less so. And a distinct benefit of those, if you just have the one rear light and it's a Varia, is that a) it'll tell you when the battery is low on the head unit but, more importantly, b) it'll say 'Radar disconnected' if the light either drops off or runs out of energy, so you *do* know that your light is still there and still working. (More precisely, you are told if it isn't.)
Garmin Varias have a mode which is always on but pulses from bright to less so. And a distinct benefit of those, if you just have the one rear light and it's a Varia, is that a) it'll tell you when the battery is low on the head unit but, more importantly, b) it'll say 'Radar disconnected' if the light either drops off or runs out of energy, so you *do* know that your light is still there and still working. (More precisely, you are told if it isn't.)