Does Mithral Armor in the form of a Chain Shirt still count as a light armor? I remember in 3.5 and Pathfinder it counted as light armor so Rogues could then use it; and often sought out the armor..
No, the armor's category does not change.
Armor (medium or heavy, but not hide), uncommon
Mithral is a light, flexible metal. A mithral chain shirt or breastplate can be worn under normal clothes. If the armor normally imposes disadvantage on Dexterity (Stealth) checks or has a Strength requirement, the mithral version of the armor doesn't.
(DMG, p. 182)