[jonmoulding]

Hello, all, new Volt owner here. (Edit to mention it's a 2011)

When driving in L, and letting off the acceleration to slow the car down- do the brake lights activate?
Having been rear ended in a bad accident in the past, I'm a bit more hypersensitive to the possibility.
Slowing down as much as the Volt can without using the brake pedal has me worried about catching other drivers off guard if there's no indication.

Not on Gen 1. The deceleration supplied by Gen 1 L is below the threshold that requires lamp illumination. Gen 2 has a paddle that does illuminate the brake lights (most of the time) because it does supply enough deceleration, but L still doesn't for the same reason as Gen 1: it's not enough to be required. Only the service brake (the wide pedal) is required to illuminate any time it's applied, regardless of how deceleration is happening.

If you're worried about it, just use the wide pedal. You'll get all the regen you would with L using the wide pedal anyway (because the Volt's braking system is cleverer that Some Others We Could Mention), and you'll already have your foot in the right place in case you suddenly need MORE braking. Like hidden ice, suicidal squirrel, bump that sets off the traction control, etc.

In my 2019 Volt, L does activate the brake lights. I confirmed this the same way Obermd described in his comment, viz. reflection off street signs at night.