Red-Eyes Darkness Metal Dragon:
By Limiting REDMD, two decks are affected, namely Hieratics and Chaos Dragons. The most broken part of Hieratics IMO is Gustav Max, it is like a Dark Strike Fighter for the deck. With REDMD at 1, players will have to wait for 4 Combo Pieces to perform their OTK, which makes the deck much more balanced as they already have access to lots of Rank 6 XYZs which are powerful in their own right.
As for Chaos Dragons, limiting REDMD will makes the Lightpulsar Combo less consistent. They still have cards like Eclipse Wyvern and Future Fusion so it shouldn’t affect them that much. I chose not to hit Future Fusion because the deck isn’t that prevalent in the OCG Meta, so I don’t think Konami will not hit Chaos Dragons directly by Banning it.
Dark Magician of Chaos:
With Monster Reborn banned, one of the best targets for this guy’s effect is gone. Also 2 Tributes are pretty hard to get in our current fast paced format, so I doubt it will see much play other than in the new Book of Spells deck. I strongly believe that Konami would unban this cards just to promote their Book of Spells archetype.