@basud, I have heard that Big Sur 11.2 has support for the new cards, but not hardware acceleration yet. So I'm hopeful it won't be too much longer.
To do: Create my signature with system and expected eGPU configuration information to give context to my posts. I have no builds.
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Most likely scenario is a big navi MPX card for the Mac Pro. I can't imagine apple will leave all the people who shelled out the cash for a Mac Pro hanging with no upgrades.
2019 16 Macbook Pro + Core v2 + Radeon VII MacOS 10.15.1
Core X Chroma + RTX 2080 Windows 10 1909
Asrock X570 Phantom Gaming ITX/TB3, Ryzen 5 3400G + Core v2 + Radeon VII Win10 1909
2020 13" Razer Blade Stealth (GTX1650Ti) [11th,4C,G] + RTX 2080 @ 32Gbps-TB4 (Razer Core X Chroma) + Win10 20H2 [build link]
(Whispering) Pssssst.... Apple already officially supports installing Big Navi hardware in a currently shipping Mac model.
@mini-i5, as cool as it is, install != supported.
MacBook Pro 16 2019 - i9 2400, 32GB memory 5500m - eGPU Razer Core X Chroma with RX580
@mymantiz_john, will it be supported on Catalina or only on Big Sur?
Macbook Pro 15" Late 2013 - Intel i7-4850HQ @ 2.3GHz - nVidia GeForce GT 750M 2GB VRAM - 16GB RAM DDR3 @ 1600MHz - Custom eGPU enclosure with JHL7440 chipset - XFX RX 560 4GB VRAM @ Sapphire VBIOS
I would think it a high certainty that Apple will support the 6XXX series. Until they have their own desktop GPU chip that outperforms AMD's best offering, for the Mac Pro to be a true "pro" Apple must offer support for the top AMD GPU's available. It's not exactly a huge deal in work hours to sort drivers out.
Driver ( kext ) is ready, Apple will support Radeon 6xxx soon.
It has been a month now without news AFAIK.
How soon is „soon“? 
Late 2016 15" MBP w/ Radeon Pro 460, 2m CalDigit TB3 cable, Mantiz Venus eGPU case, NVidia GTX 1080ti.
@jimmcq, Catalina recognises the GPU but with no hardware acceleration, just like Big Sur does... I wonder when it's gonna be supported
Macbook Pro 15" Late 2013 - Intel i7-4850HQ @ 2.3GHz - nVidia GeForce GT 750M 2GB VRAM - 16GB RAM DDR3 @ 1600MHz - Custom eGPU enclosure with JHL7440 chipset - XFX RX 560 4GB VRAM @ Sapphire VBIOS
Still the same with macOS Big Sur Version 11.2 (20D62). Recognizing but not working!
Mac mini | 6 Core | 64GB | 2TB |
MacBook Pro M1 | 16GB | 2 TB |
3x Mantiz Venus 2 / Radeon Frontier Edition | 1x Razer Core / Radeon RX 6800 |
OS X 11.2
There's some progress with Big Navi in beta 11.3. While they are still not working, the RX 6000 series hardware ID [0x73BF1002] are in AMDRadeonx6000Framebuffer.kext. As @mac_editor has observed over the years, system update .4 is the one with major changes so my bet is that's when we see full support for RX 6800/6900 XT.
• external graphics card builds
• best laptops for external GPU
• eGPU enclosure buyer's guide
2021 15" ASUS TUF Dash F15 [11th,4C,H] + RTX 3080 @ 32Gbps-TB4 (AORUS Gaming Box) + Win10 2004 // my 3rd RTX 3080 build [build link]
Let's hope for 11.4 then!
Late 2016 15" MBP w/ Radeon Pro 460, 2m CalDigit TB3 cable, Mantiz Venus eGPU case, NVidia GTX 1080ti.