it isn't set in the DB, it is hard-coded in MP and DCS. It was once the case, long ago, that IL-2 would crash if it saw waypoints less than 200 m apart. It doesn't seem to matter now, so maybe we can relax this requirement a bit.
You are right, Zoi. You can plot as many waypoints as needed no matter they are out of the path you want to move along. Then you have to edit each waypoint, writing the right coordinates of each corner you want to bend. Takes a little more time for planner but the result in the mission looks pretty good.