Shadowflame wasn't designed to be an anti shield item. It is a magic pen item for champions that cannot proc any of the other pure damage items (horizon, Lich, etc.) with an anti-shield cherry on top.
We found that gating the full item functionality behind an opponent team composition having shielders, when it's meant to be a core item did not satisfy those users.
These champions are often trying to nuke champions through a defensive shield like Janna, lulu, locket etc.
In those situations against a full hp ~15-1800 hp target, getting the max pen can be significant. 7 mpen can be a surprising amount of damage and it also has good effect against bruisers who are frequently at the max hp threshold and taking ambient damage.