Not used at all but no box. Long cage.

Shimano Deore XT RD-M786 10-speed rear derailleur.
It features Shadow Plus clutch technology, which reduces chain slap and prevents chain drops on rough terrain.
The derailleur has a narrow profile, reducing the risk of snagging on trail obstacles.
It is a 10-speed component designed for mountain bikes, gravel bikes, and e-bikes.
The construction is stiffer and more machined, which increases durability and improves shift quality.
The model shown appears to be the black color variant with a GS (medium) or SGS (long) cage option.