What does Thunderhorn Cleansing reward?

Thunderhorn Cleansing gives Thunderhorn Cloak outright, 700 experience.

Where does Thunderhorn Cleansing start?

Mull Thunderhorn in Mulgore offers it.

Its level has not been recorded yet; it shows once a player accepts it with the addon running.

No. The quest exists in Classic Era as well.

World of Warcraft: Forever · quest

A quest in World of Warcraft: Forever, offered by Mull Thunderhorn in Mulgore. Recorded by 1 player running the addon.

Level and objectives show once a player with the addon accepts the quest.

Item	Given	Item level	Vendor sell
Thunderhorn Cloak	outright	8	30c

What does Thunderhorn Cleansing reward?: Thunderhorn Cleansing gives Thunderhorn Cloak outright, 700 experience.
Where does Thunderhorn Cleansing start?: Mull Thunderhorn in Mulgore offers it.
What level is Thunderhorn Cleansing?: Its level has not been recorded yet; it shows once a player accepts it with the addon running.
Is Thunderhorn Cleansing new in WoW Forever?: No. The quest exists in Classic Era as well.

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