Keep It Simple/Stupid (KISS)
"As simple as possible, as complex as necessary"
KISS is keeping the road geometry as simple as possible, without losing the essential driving instructions.
More info, see the KISS page.
Filter lanes are not mapped.
Filter lanes don't fit the KISS principle because they are the cause of very complex situations that are hard to manage, while they don't add a lot of value in most routing instructions. Short segments and extra splits can have a negative influence on routing (extra penalty, incorrect average speeds, ...).
Of course, there is always an exception. If the choice of a filter lane must be made more than 50 meter before coming to the crossing AND the main road and filter lane are divided by obstacles (concrete blocks, ...), it should be mapped.