Rogue talent · row 4 of Combat, from level 25 (20 with Talented 5/5) · Physical

Dual Wield Specialization

Increases the damage done by your off-hand weapon by 5/10/15/20/25%.

Game files · build 1.60.1.70245Read from the WoW Forever beta client, build 1.60.1.70245.

Compared with Classic

Classic tooltip

Increases the damage done by your offhand weapon by 10%.

Data	Classic	Forever
effect 1 value	10	50

What Blizzard said about Dual Wield Specialization, each line checked against the files.

Matches the files
Dual Wield Specialization no longer grants hit to your off-hand attacks.
The files The hit effect (aura 54, 10%) becomes a dummy in the 70245 hotfix, and the tooltip offers off-hand damage and Rage instead (spell 23584).
Dual Wield Specialization Warrior updates, October 2
Changed since
Dual Wield Specialization: Now also increases chance to hit with off-hand weapons.
The files In 70170 it added 2-10% off-hand hit (aura 54). The Oct 2 update removed that: in the 70245 hotfix the effect is a dummy and the tooltip offers off-hand Rage instead (spell 23584).
Dual Wield Specialization Class deep dives: Priest and Warrior

69876 Sep 16First beta build we archived. No changes in the client files since.