While it has been an iOS exclusive for quite some time now, Siri will be coming to macOS starting with the upcoming iMac Pros and MacBook Pro versions. The ARM A10 Fusion chip will enable an always-on functionality that allows Siri to work even when the devices are completely shut off.
https://www.notebookcheck.net/Apple-will-add-ARM-A10-Fusion-chip-for-always-on-Siri-functionality-in-iMac-Pro-and-MacBook-Pro.264202.0.html