Finally!!
Better late than never. Who ever would have thought one year ago that the cMP would be getting so much love from Apple.
I'm about to install 10.14.4. I have the same card as you so hopefully will see the same result. Would be interesting to see reports from folks with other cards (or even different flavors of the Sapphire RX580).
Edit: Indeed it has been fixed. I can confirm the same "DisplayPort" device now showing up in System Profiler > Audio. I have a Sapphire Pulse RX580 connected to a Dell P2715Q.
For the record, I had never installed any of the kexts or other hacks to get this feature working before, so this fix is 100% Apple's doing. Also, I'm still on firmware 140.0.0.0.0 so the fix is in the OS itself, and not in 141.0.0.0.0. I don't actually use DP audio with my current setup, so I had pretty much forgotten about this bug. I had given up on Apple fixing it since I and several others filed detailed reports during the Mojave beta period. I wonder if Apple always intended to fix it but just considered it low enough a priority to wait until a later point release. In any case, yet another longstanding bug finally fixed in the cMP's twilight years.
I am really curious though if people with other supported GPUs have it fixed also, or if Apple only fixed it for the 560 & 580.
Nice find bcjenkins!
Edit 2: Here's how it shows up in System Profiler > PCI.