While Apple did announce official support for Thunderbolt 3 eGPU implementations, a footnote nestled deep in the High Sierra preview page declares that user support in a non-beta fashion won't be available to users until Spring 2018.

Apple's note on the matter springs from the "Virtual reality for Mac" section. In the section, Apple notes that the HTC Vive VR headset is supported on Mac with "the new iMac with Retina5Kdisplay, the new iMacPro coming in late 2017, or any supported Mac paired with an external GPU."

Following that sentence, is a link to footnote 3 on the same page, noting that the external GPU expansion capability is "planned for spring 2018."

AppleInsider has briefly examined eGPU support in macOS High Sierra, and has discovered that third-party hardware and video cards work the same as the developer's kit should. There are notable pre-release testing issues according to Apple's developer notes, including:

Lack of support for mirrored displays

No "clamshell mode" support

No support for HDMI audio

AutoLogin failure when EGPU is attached

No acceleration of internal display on iMac, MacBook Pro

60W of charging power, not 87W

No support for USB-C displays

No sleep support for attached mac — but display sleep is fine

Apple's developer kit for external Thunderbolt 3 GPU testing costs $599 and includes a Sonnet external GPU chassis with Thunderbolt 3 and 350-watt power supply, an AMD Radeon RX 580 8-gigabyte graphics cards, a Belkin USB-C to 4-port USB-A hub, and a promo code for $100 towards the purchase of an HTC Vive VR headset. Macs with Thunderbolt 2 ports are compatible with third-party eGPU enclosures, but Apple lists it as an "unsupported configuration."

To go along with Apple's VR and eGPU initiative, Valve launched its own first beta of SteamVR for the Mac on Monday, giving early testers the ability to sample virtual reality experiences on compatible Macs.