Apple's iMac Pro has gained support for "Hey Siri" with macOS Mojave.
After the MacBook Pro 2018 gaining support for it, the iMac Pro has gotten "Hey Siri" support. The macOS Mojave update enables it. Both the iMac Pro and MacBook Pro 2018 have the T2 chip in which allows for this functionality.
Apple have confirmed it has been added with Mojave.
Hey Siri first came to the iPhone and has been added to recent Apple desktops and other devices too, like the Apple Watch and HomePod. The devices that support Hey Siri are outlined in this document from Apple.