This had me worried the first few times it happened. Then I found out it is a warning to mechanics/tow truck drivers/first responders that the high voltage system is energized.
It is only on if the car is turned off and parked and the high voltage system has turned on to charge the 12V battery. When you're driving or just have turned on the car while parked, the light goes off.
So the only occasion the light might be annoying is if you're taking a nap in the car at night. And then you could just throw a towel on top of it.
You expect to have the high voltage system energized when driving, otherwise the car wouldn't go anywhere. There would be no purpose for this light to go on.
The light is specifically to indicate that the high voltage system is on when the car is parked. It's a safety warning for that situation.
In Toyota, at least, the traction battery is completely disconnected (via a relay) when the car is off. The 12V battery is needed to power anything on the 12V system up until the car starts-- that includes the car's computer, which is what (after doing all its self-checks and whatnot) activates that relay and connects the traction battery to everything else in the car.
The traction battery isn't always connected probably mostly for safety reasons (having 400-someodd volts energized across the whole car even when it doesn't need it isn't great), but that also keeps it from getting excessively drained if something in the car malfunctions. It's pretty cheap to replace a 12V lead-acid battery if it's overdischarged after you left the lights on... the big hybrid battery, not so much.