The Facebook team responsible for Instagram had to use an older version of Xcode (Apple's developer tool) to compile and submit the update to the App Store. For apps to work with the new screen size of the iPhone XS Max and XR, they must be compiled and submitted using Xcode 10, linking against the iOS 12 SDK.
From what we've been able to gather from sources, the Facebook team had to distribute the app with an older version of Xcode because of a common crash that can occur with apps compiled using the iOS 12 SDK but running on iOS 9, a system version which a large number of users of Instagram are still running.
The only bug referenced is a bug with asset catalog. So the third solution for the instagram developers can be to avoid using Asset catalog for features that are not compatible with iOS 9.
"Another, better solution, would be for Apple to fix the bug, either by updating the SDK or issuing an update to iOS 9."
It's been a long time since Apple stopped caring about fixing bugs.
Considering they don't even provide security updates to iOS versions other than the current one, praying for an update to iOS 9 is just a joke.