What does Stranglethorn Fever reward?

Stranglethorn Fever gives Medicine Blanket outright, 5,850 experience.

Where does Stranglethorn Fever start?

Fin Fizracket in Stranglethorn Vale 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 Fin Fizracket in Stranglethorn Vale. 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
Medicine Blanket	outright	45	48s 53c

What does Stranglethorn Fever reward?: Stranglethorn Fever gives Medicine Blanket outright, 5,850 experience.
Where does Stranglethorn Fever start?: Fin Fizracket in Stranglethorn Vale offers it.
What level is Stranglethorn Fever?: Its level has not been recorded yet; it shows once a player accepts it with the addon running.
Is Stranglethorn Fever 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 · Stranglethorn Vale