What does Leonid's Letter reward?

Leonid's Letter gives Mug of Muddled Memories, Misplaced Shooter, Tim's Lost Rib outright.

What level is Leonid's Letter?

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

Yes. Its quest id is not in Classic Era, so it is part of what Forever adds.

World of Warcraft: Forever · quest · new since Classic Era

A quest in World of Warcraft: Forever in Undercity. Recorded by 1 player running the addon.

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

Item	Given	Item level
Mug of Muddled Memories	outright	12
Misplaced Shooter	outright	12
Tim's Lost Rib	outright	12

What does Leonid's Letter reward?: Leonid's Letter gives Mug of Muddled Memories, Misplaced Shooter, Tim's Lost Rib outright.
Where does Leonid's Letter start?: It was picked up in Undercity.
What level is Leonid's Letter?: Its level has not been recorded yet; it shows once a player accepts it with the addon running.
Is Leonid's Letter new in WoW Forever?: Yes. Its quest id is not in Classic Era, so it is part of what Forever adds.

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