Is there any way to detect iOS 9 low power mode programmatically?

Yes, you can either add an observer or query it directly.

https://developer.apple.com/library/archive/documentation/Performance/Conceptual/EnergyGuide-iOS/LowPowerMode.html

Here is a code snippet to check for low power mode. Note that this feature requires iOS 9 or above. If you are not targeting an older version, you can do away with the version check.

Swift 4.2

if #available(iOS 9.0, *) {
    if ProcessInfo.processInfo.isLowPowerModeEnabled {
        <#Do low power stuff#>
    } else {
        <#Not in low power mode#>
    }
}

Doc

https://developer.apple.com/documentation/foundation/processinfo/1617047-islowpowermodeenabled

Swift 3.0

if #available(iOS 9.0, *) {
    if ProcessInfo.processInfo().lowPowerModeEnabled {
        <#Do low power stuff#>
    } else {
        <#Not in low power mode#>
    }
}

Swift 2.2

if #available(iOS 9.0, *) {
    if NSProcessInfo.processInfo().lowPowerModeEnabled {
        <#Do low power stuff#>
    } else {
        <#Not in low power mode#>
    }
}