Apple's fifth-generation iPad, which launched in March 2017, does not support tetherless “Hey Siri” functionality even though the device features an embedded M9 motion coprocessor. As first noted by AppleInsider, the 9.7-inch iPad Pro is the only Apple tablet the feature is currently available on.
An always-on Siri requires that the device be outfitted with a low-power motion coprocessor embedded into the main chip. This lets iOS listen for the “Hey Siri” wake up phrase on battery power, in the background, without waking up the main power-hungry processor.
TUTORIAL: “Hey Siri” not working? Here's your troubleshooter!
“Hey Siri” made its debut with iPhone 6s, the first Apple device to embed the motion coprocessor in the main processing chip. All iPhones from iPhone 6s onward support “Hey Siri” on battery power, including iPhone SE and the latest iPhone 7 series.
The 12.9-inch iPad Pro is another Apple tablet without support for tetherless “Hey Siri” despite the inclusion of the M9 chip. It's unclear at the moment if Apple could eventually enable untethered “Hey Siri” in software on the new iPad and the 12.9-inch iPad Pro.