Where is Sacred Duty in the Paladin talent tree?

Protection tree, row 2, column 4. You need 5 points spent in Protection before you can learn it.

Is Sacred Duty new in WoW Forever?

Yes. Sacred Duty has no Classic 1.12 counterpart.

Rank	Effect	Source
1	Reduces the cooldown of your Divine Shield, Divine Protection, and Templar's Bulwark spells by 30 sec, and reduces the duration of Forbearance by 5 sec.	client
2	Reduces the cooldown of your Divine Shield, Divine Protection, and Templar's Bulwark spells by 60 sec, and reduces the duration of Forbearance by 10 sec.	client

Tree: Protection, row 2, column 4. Needs 5 points in Protection first. The earliest you can finish all 2 ranks is level 16 if you put every point into this tree.
Competes with: Precision, Guardian's Favor in the same row.

Sacred Duty is new in WoW Forever. Nothing in Classic 1.12 does this for Paladins.