How Can I Start the Arms Dealer Strike as Part of the Leviathan´s Breath Quest in Season 9?
According to @Sadak in the comments, as well as this reddit thread, this bug as been resolved in Update v2.7.1.
On January 9th, 2020, Bungie released their weekly news article after the holiday season had come to pass. This news article contained an official statement that they are currently tracking this issue, as well as various other issues that have risen
TRACKING KNOWN ISSUES
Over the past few weeks, a few more issues have surfaced that our Player Support team has been tracking. Below is a list of the latest known issues that were reported in our #Help Forum:
- The Leviathan's Breath Exotic Bow currently cannot be acquired due to an issue with 'The Arms Dealer' strike where it can only be completed in Season 8.
The article did not state if a fix will be applied for this quest in this update, but at least we have something official from Bungie regarding this bug.
There is currently a bug around this quest within Season of the Dawn. There are multiple reports where players are unable to finish this quest once Season of the Undying ended.
Bungie released a help guide on what is being removed in the new season. The guide states
Once Season of Dawn begins, the following list of items will no longer be available to players from Season of the Undying:
- The Leviathan's Breath exotic quest if players didn't own the Season Pass. This will be available to players who own a Season Pass either now or in the future.
So based on this information, you should be able to complete this quest in Season of the Dawn as long as you have the season pass. The reports I linked state that Bungie is aware of this issue, and should hopefully implement a fix for this