Supports are always underleveled.
Patch 5.22 introduced a new mechanic to gold: After 12 minutes, the two players with the lowest amount of gold on each team gain additional GP5 which increases by 0.1 each minute.
Why can't the same be done to XP? After 12 minutes or 15 minutes of 18 minutes or w/e the player with the lowest level gets passive xp.
Supports are often still level 15 at 40 minutes. This needs to be fixed. Be it by adding passive XP, giving sightstone bearers xp for killing wards, make the explorer mastery grant a bit of passive XP gain, something. Anything.