Warrior talent · row 1 of Protection, from level 10 (5 with Talented 5/5) · Physical

Iron Will

Reduces the duration of Stun and Fear effects inflicted on you by 3/6/9/12/15%.

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

Compared with Classic

Classic tooltip

Increases your chance to resist Stun and Charm effects by an additional 15%.

What Blizzard said about Iron Will, each line checked against the files.

Matches the files
Iron Will moved to the Row 1 of Protection (Was Row 2 of Fury).
The files The Oct 2 server hotfix (read from the 70245 client cache) added a new Iron Will node (113570, 5 ranks, 3 to 15% curve) in Protection row 1 and gave its old Fury row 2 node to Lingering Rage.
Iron Will Development Notes, October 1
Matches the files
Iron Will moved to the Row 1 of Protection (Was Row 2 of Fury).
The files The 70245 hotfix adds a 5-rank Iron Will node (113570) in Protection row 1, and its old Fury row 2 slot becomes Lingering Rage.
Iron Will Lingering Rage Warrior updates, October 2
Changed since
Iron Will: Moved from Protection to row 2 of Arms. Now reduces duration of Stuns instead of increasing resistance to them.
The files In 70170 it sat in Fury row 2 (the line says Arms) and cut Stun duration by 3-15% (tooltip). Blizzard's Oct 2 update moved it to Protection row 1, which the 70245 hotfix shows (new node 113570).
Iron Will Class deep dives: Priest and Warrior