What does Heirloom Weapon reward?

Heirloom Weapon gives a choice of Heirloom Axe, Heirloom Hammer, Heirloom Dagger, Heirloom Sword, 875 experience.

Where does Heirloom Weapon start?

It was picked up in Tirisfal Glades.

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 in Tirisfal Glades and turned in to Coleman Farthing. Recorded by 1 player running the addon.

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

Item	Given	Item level	Vendor sell
Heirloom Axe	as a choice	15	6s 83c
Heirloom Hammer	as a choice	15	6s 88c
Heirloom Dagger	as a choice	15	6s 85c
Heirloom Sword	as a choice	15	6s 90c

What does Heirloom Weapon reward?: Heirloom Weapon gives a choice of Heirloom Axe, Heirloom Hammer, Heirloom Dagger, Heirloom Sword, 875 experience.
Where does Heirloom Weapon start?: It was picked up in Tirisfal Glades.
What level is Heirloom Weapon?: Its level has not been recorded yet; it shows once a player accepts it with the addon running.
Is Heirloom Weapon 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 · Tirisfal Glades