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Shifting at low RPM in and of itself isn't bad for anything. The only thing bad in what you describe would be if you are lugging the engine at low RPM (basically below 2k). However, if you're very gentle on the throttle at those low RPMs, then that isn't an issue either. In heavy traffic, I often keep the car in 2nd, which has the engine well below 2k RPM, but it is either just idling along, or I'm using light throttle, so it doesn't matter.
Low RPMs aren't bad for anything. Lugging the engine at low RPMs is.
Also does 15L/100km really translate to 15 MPG (US)? If so, something is horribly wrong with your car. I can't even get that low mileage in a day of autocrossing and towing a tire trailer. You should be getting closer to 25 MPG (10L/100km) in traffic, especially in how you're trying to drive efficiently.
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