Ultimately, the Shadow Magic Sorcerer, like other Sorcerous Origins, is really defined by the individual’s spell choices more than the subclass itself, which allows for flexible building and flavor. The real worth however comes from continuing Disadvantage on spells with repeated saves, which Heighten does not do. The main gimmick of the subclass is a shadowy summon at sixth level that imposes Disadvantage on saves vs your spells for the same price as the Heighten spell metamagic, which is excellent value against a single enemy that you continue to hammer with spells, but less so in battles against numerous foes. It has a few tools that interact with darkness and dim light, but as a whole, even if you spent the entire campaign in well-lit environments, this subclass would do quite well.
The Shadow Magic Origin Sorcerer is not really all that concerned with shadows or darkness.
