Where is Improved Drains in the Warlock talent tree?

Affliction tree, row 2, column 3. You need 5 points spent in Affliction before you can learn it.

Is Improved Drains new in WoW Forever?

Yes. Improved Drains has no Classic 1.12 counterpart.

Rank	Effect	Source
1	Increases Health drained, damage done, or Mana drained by your Drain Life, Drain Soul, and Drain Mana spells by 7%.	client
2	Increases Health drained, damage done, or Mana drained by your Drain Life, Drain Soul, and Drain Mana spells by 13%.	client
3	Increases Health drained, damage done, or Mana drained by your Drain Life, Drain Soul, and Drain Mana spells by 20%.	client

Tree: Affliction, row 2, column 3. Needs 5 points in Affliction first. The earliest you can finish all 3 ranks is level 17 if you put every point into this tree.
Competes with: Grim Reach, Soul Harvest in the same row.

Improved Drains is new in WoW Forever. Nothing in Classic 1.12 does this for Warlocks.