On top of Strikes, Crucible, and Gambit, you can also now get it from Vex Offensive.
Edit - Quote and link.
[quote]Rewards
The Season Pass weapons Pluperfect and Temporal Clause now have a chance to drop from Vex Offensive after you have obtained them.[/quote]
[url]https://www.bungie.net/en/Explore/Detail/News/48420[/url]