Magsafe "L" light takes few seconds before turning off after being unplugged
It's nothing more than capacitor discharge.

The larger the capacitance, the longer it takes to dissipate once the power source is removed.

It's a power smoothing feature, not just in laptop chargers but many other electrical items.
Without it, small power fluctuations in the mains would cause computer glitches or crashes - or in simpler structures like LED lighting, flickering.