Priest talent · row 2 of Shadow, from level 15 (10 with Talented 5/5) · Physical

Shadow Affinity

Reduces the threat generated by your Shadow spells by 10/20/30%.

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

Compared with Classic

Classic tooltip

Reduces the threat generated by your Shadow spells by 8%.

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

Matches the files
Shadow Affinity: Threat reduction to Shadow spells increased slightly.
The files Shadow threat reduction goes from 8/16/25% in Classic to 10/20/30% in 70245 (talent rank values, spell 15318).
Shadow Affinity Class deep dives: Priest and Warrior

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