Apple details eGPU support in macOS 10.13.4 High Sierra, notes no Nvidia support
The release of macOS 10.13.4 brings with it official external GPU support by Apple, but the company is recommending specific enclosures/chassis and cards, excluding ones some people might hope to use.

In a support page posted on late Thursday, the company suggests several AMD video cards. These include the Radeon RX 570 and RX 580, the Radeon Pro WX 7100 and 9100, and finally the Radeon RX Vega 56, 64, and Vega Frontier Edition Air. Suggested enclosures come from companies like PowerColor, Sonnet, and OWC -- but others can work as well.
Support is not limited to any specific card vendor. As long as the card mostly complies with the reference specification, they are able to be used in an eGPU enclosure.
The list notably excludes any cards from Nvidia. Nvidia does have the "web driver" for its PCI-E that it updates after every macOS revision, but at present that isn't sufficient to run the cards in an enclosure, without third-party hacks applied, and even that can be problematic.
During the beta process, some cards that worked during Apple's initial testing of eGPU support are no longer supported, such as the AMD RX 560. Mac owners must also use Thunderbolt 3, since official support for earlier versions of Thunderbolt was also dropped in the beta period.
Apple notes that people can connect more than one eGPU, though people should use direct connections whenever possible instead of daisy-chaining. Apple's enclosure recommendations all have 87W of charging power available to the host machine, and 13-inch MacBook Pros from 2016 or later should always have eGPUs plugged in on the left-hand side to guarantee maximum bandwidth. Any VR headsets should be plugged into the eGPU.
To check whether cards are working properly, users should open the macOS Activity Monitor, then select Window and GPU History. Once a session is finished Apple notes that the new chip icon in the Mac's menu bar should be used to disconnect it.

The macOS 10.13.4 update also brought with it support for Business Chat in Messages, and a variety of bugfixes, such as for graphics problems impacting apps on the iMac Pro.

In a support page posted on late Thursday, the company suggests several AMD video cards. These include the Radeon RX 570 and RX 580, the Radeon Pro WX 7100 and 9100, and finally the Radeon RX Vega 56, 64, and Vega Frontier Edition Air. Suggested enclosures come from companies like PowerColor, Sonnet, and OWC -- but others can work as well.
Support is not limited to any specific card vendor. As long as the card mostly complies with the reference specification, they are able to be used in an eGPU enclosure.
The list notably excludes any cards from Nvidia. Nvidia does have the "web driver" for its PCI-E that it updates after every macOS revision, but at present that isn't sufficient to run the cards in an enclosure, without third-party hacks applied, and even that can be problematic.
During the beta process, some cards that worked during Apple's initial testing of eGPU support are no longer supported, such as the AMD RX 560. Mac owners must also use Thunderbolt 3, since official support for earlier versions of Thunderbolt was also dropped in the beta period.
Apple notes that people can connect more than one eGPU, though people should use direct connections whenever possible instead of daisy-chaining. Apple's enclosure recommendations all have 87W of charging power available to the host machine, and 13-inch MacBook Pros from 2016 or later should always have eGPUs plugged in on the left-hand side to guarantee maximum bandwidth. Any VR headsets should be plugged into the eGPU.
To check whether cards are working properly, users should open the macOS Activity Monitor, then select Window and GPU History. Once a session is finished Apple notes that the new chip icon in the Mac's menu bar should be used to disconnect it.

The macOS 10.13.4 update also brought with it support for Business Chat in Messages, and a variety of bugfixes, such as for graphics problems impacting apps on the iMac Pro.
Comments
DisplayLink acknowledged the problem back when the first beta was released, but has not been able to provide a fix.
I have no love for NVidia so from that standpoint i dont care. What i find strange is peoples obsession with NVidia. Niether AMD nor NVidia produce perfect cards nor perfect software. However AMD is far more open with their software which in my mind is a huge plus.
I suspect a lot is going on here. TB 1 in my mind was a proof of concept and even TB 2 was of likited interest. As such i see the USB-C port as Apples long term play with TB.
There are likely other factors with this exclusion of hardware, some of those are why i have an HP ENVY laptop right now. In any event nothing would stop Apple from supporting older machines once the kinks are worked out here.
Now do i believe that Apple will revisit this old hardware? Nope; in my opinion they have regressed to past behavior of screwing over the custome with high prices on outdated hardware. Im actually hoping we see massive changes to the Mac line up real soon. If not it could be sometime before i return as a Mac customer.
I think apple is only releasing eGPU support so it doesn't lose the VR video creators. Hooking up a highend AMD card to a macbook pro will probably give acceptable performance for some. Gaming over the eGPU is 'ok', some frame dropping will occur, but then again, there aren't that many games on the mac that stress the GPU that much, unless you force ultra quality on 4K.
A lot of the Machine Learning tools use Cuda, so people buy Nvidia. But getting GPU support for some ML tools on OSX can be troublesome as well.
Since 10.13.3 the Nvidia web drivers give a lot of issues. So that won't be resolved with eGPU support.
Maybe eGPU support is a stopover measure like the iMac pro, until we get a decent modular Mac Pro again.
CUDA support. Otherwise, I don't think most people would care all that much.
Yea, TB3 is really the first version that kinda fulfills the external bus promise. As nice as the cylinder Mac Pro is, it was too early a move in that direction by Apple... bleeding edge. I think if they had introduced the cylinder concept now with TB3 and some external GPU boxes, it would be more accepted, even by the real pros. I think most of them would still rather have a modern cheese-grater, though.
Yea, this is the big dilemma I face right now. Aside from the iMac Pro (currently, out of my budget), Apple just has no TB3 machine I want. I was thinking of getting a cMP or 2015 MBP, but now this has me re-thinking that. I'm guessing the new Mac Pro (even if it comes soon), is going to be quite expensive. I'm not sure if Apple will really fix the MBP any time soon. Possibly, they'll update the Mini, but I doubt it will be quad-core.
Apple just has nothing in the middle. They have really expensive high end (finally). And, then they have a bunch of coffee-shop jockey stuff. I need a reasonably powerful, hopefully somewhat quiet, prosumer machine that falls somewhere into Apple's non-pro pricing range.
Possibly, but I hope not. I think the eGPU concept is great to get the best of both worlds in many cases. I don't always need a hefty space-heater/wind-tunnel GPU running, but sometimes I do. Good eGPU support would be great for laptop people, system expansion, or just split use-cases. It's actually one of the few 'innovations' in computers I've been excited about for a while.