'Y" level does not matter when or where starting a block pillar. When pillaring up from natural terrain at any starting 'Y' level, the sky light values eventually shrinks to zero. When removing pillar, the original terrain block the pillar was started on will not return to sky light level 15 but hold a light value of 14 or lower.

Tested on more than one seed. Also tested in and out of spawn chunks. Tested from sea level and from mountains/hills. 

Pictures show where sky light levels fade away to nothing. Sky/block light on the top block with nothing placed above it has a light value of zero.