From a new support document on TP-Link's website via Reddit:
Due to changes in our roadmap, the Kasa Smart Plug Mini (HS105) smart plug will not offer support for Apple HomeKit. Despite the change in direction for this specific SKU, our team is committed to the development of innovative smart home solutions that will continue to delight our customers around the globe.TP-Link originally planned to add HomeKit support to the smart plug in early 2019, a promise that it has now broken.
Fortunately, HomeKit-enabled smart plugs are quite common, with a wide variety of options available from brands such as Eve, iDevices, iHome, and Wemo. They can be used to conveniently turn on and off TVs, lamps, coffee machines, and other household objects with an iPhone, iPad, Mac, or Apple Watch.