Old Thornmail was better at countering lifesteal than New Thornmail
There are very very few points in the game where New Thornmail outclasses Old Thornmail's ability to counter lifesteal.
In the attached table, Damage dealt for Old Thornmail is calculated as follows: [15%*Premitigation Damage] + [25%Armor] - [25% LifestealPostmitigation Damage]
Damage dealth for New Thornmail is calculated as follows: [25] + [10%Armor] - [25% LifestealPostmitigation Damage] * [60% Grievous Wounds]
*For simplicity, I assume all damage is bonus damage, since base armor changes per level and by champion.
I don't know how this can be viewed as anything other than a massive nerf.