Yeah I'm still not buying into the whole "Supports are screwed by the system :(((" argument. Even if support players are getting less exp than the rest, it seems like it would be remedied by the underdog exp bonus to keep them caught up to their friends. The idea that your team is so good they don't need healing seems kind of flawed considering they track statistics like % of team damage taken healed. We don't officially know how they determine rank gained but I would bet on them using stuff like that. Healers also have other things they can do besides healing to contribute such as assists (Mercy for defensive, Zen for offensive) or racking in a ton of objective time (Lucio especially, but Ana makes a good cart pusher too) If anyone of the supports really suffered from the rank system it'd probably be Symmetra, but if the enemy team gets the first point (which the usually will) most people swap off Symmetra anyway due to diminishing returns on her usefulness. These kind of comments always assume that the healer is doing everything perfectly, which honestly isn't always the case. Believe it or not, it IS possible to have a bad support on your team.
Also since on average 1/3 of the team are supports and the other 2/3 are dps/tanks then it makes sense to have less supports in top 500. Of course there is a bias towards attack heroes which honestly seems expected. If non supports are picked twice as much as supports, a well rounded player might have 1 support in their top 3 due to preferences. I personally think the rest of the team is required to perform a lot more than the supports are, and in general are more skill demanding. None of the supports have a particularly high skill cap in terms of execution, except Zenyatta. If anything they rely a lot more on game sense, which is a general trait anyone improves to go up in ranks. This comment isn't meant to undermine the importance of supports, as they're crucial to winning any game. It just happens that almost any big play made in a game is done by a non-support, while support ults counter those attempts.