History of Anonymous Autumn

Anonymous Autumn is a Legendary Sidearm with a Lightweight Frame (360 RPM, 14-round magazine).

Overview

Destinypedia says it debuted in the Forsaken year as a Solar weapon and is Arc as of Episode: Revenant; the Destiny Game Wiki lists it as Arc, Season 25. The Game Wiki's curated roll is Extended Barrel, Ricochet Rounds, Eddy Current, Swashbuckler and the One Quiet Moment origin trait.

Acquisition

Both sources describe it as a Crucible reward: matches and rank-up packages from Lord Shaxx.

The two opened sources (Destinypedia and the Destiny Game Wiki) disagree on some details such as the season of introduction, and neither states whether the weapon can be crafted or has an Adept version, so those are treated as not stated. Bungie's in-game source text reads "Random Perks: This item cannot be reacquired from Collections."