Where is Lightning Overload in the Shaman talent tree?

Elemental tree, row 5, column 3. You need 20 points spent in Elemental before you can learn it, and Elemental Alacrity must be at full rank first.

Is Lightning Overload new in WoW Forever?

Yes. Lightning Overload has no Classic 1.12 counterpart.

Rank	Effect	Source
1	Gives your Lightning Bolt and Chain Lightning spells a 10% chance to cast a second, similar spell on the same target at no additional cost that causes half damage and no threat.	client
2	Gives your Lightning Bolt and Chain Lightning spells a 20% chance to cast a second, similar spell on the same target at no additional cost that causes half damage and no threat.	client
3	Gives your Lightning Bolt and Chain Lightning spells a 30% chance to cast a second, similar spell on the same target at no additional cost that causes half damage and no threat.	client

Tree: Elemental, row 5, column 3. Needs 20 points in Elemental first. The earliest you can finish all 3 ranks is level 32 if you put every point into this tree.
Prerequisite: Elemental Alacrity at 3/3.
Unlocks: Elemental Fury (requires Lightning Overload at full rank).
Competes with: Elemental Reach, Call of Thunder in the same row.

Lightning Overload is new in WoW Forever. Nothing in Classic 1.12 does this for Shamans.