What does Panther Mastery reward?

Panther Mastery gives Panther Hunter Leggings outright.

Where does Panther Mastery start?

Sir S. J. Erlgadin in Stranglethorn Vale offers it.

The quest log lists it at level 40.

No. The quest exists in Classic Era as well.

World of Warcraft: Forever · quest · level 40

A level 40 quest in World of Warcraft: Forever, offered by Sir S. J. Erlgadin in Stranglethorn Vale. Recorded by 1 player running the addon.

Pay shows once a player with the addon opens the quest.

Item	Given	Item level	Vendor sell
Panther Hunter Leggings	outright	40	54s 3c

What does Panther Mastery reward?: Panther Mastery gives Panther Hunter Leggings outright.
Where does Panther Mastery start?: Sir S. J. Erlgadin in Stranglethorn Vale offers it.
What level is Panther Mastery?: The quest log lists it at level 40.
Is Panther Mastery new in WoW Forever?: No. The quest exists in Classic Era as well.

What sells at this level, and what to keep from these rewards, is in the level 30 to 40 gold guide.

Source: what players running the addon saw in the quest windows; the quest text itself stays in the game. Every quest seen · Stranglethorn Vale