Warlock talent · row 6 of Affliction, from level 35 (30 with Talented 5/5) · Physical

Shadow Mastery

Increases the damage dealt or life drained by your Shadow spells by 1/2/3/4/5%.

Game files · build 1.60.1.70291Read straight from the WoW Forever beta client.

Compared with Classic

Classic tooltip

Increases the damage dealt or life drained by your Shadow spells by 2%.

Data	Classic	Forever
effect 3	% spell modifier 1	removed

What Blizzard said about Shadow Mastery, each line checked against the files.

Matches the files
Shadow Mastery: Damage bonus reduced.
The files Shadow Mastery (18271) gives 1-5% Shadow damage; Classic gave 2% per point (10% at 5).
Shadow Mastery Class deep dives: Rogue and Warlock

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