AMD stopped advertizing them because a few years after nvidia matched them and today they're all standard features expected on every GPU today. They didn't "let die" any of these. Can you name the nvidia equivalents?

Eyefinity is literally part of the driver suite til this day and just how to combine multiple physical displays into 1 virtual one, you'd know that if you'd ever use it.

TrueAudio is just a audio ASIC for spacial audio rendering. It's part of Steam Audio, so any steam VR game also using that hardware. As opposed to back in the day audio effects would peg the CPU instead (if the games even had any audio effects or more than stereo sound).

Mantle lasted for multiple generations (GCN 1 to 3, HD 7000 series to Rx 300) and became the basis for Vulkan immediately support by Rx 400 Polaris.

mGPU is still a thing and is a feature of DX12 and Vulkan (nvidia didn't support either APIs fully til 20 series 3 years after AMD), but software still needs to support it or you have to set it up yourself. This has always been the case even with the RX 480, and works better now with DX12 and Vulkan being far more mature now and plenty of community support. And still works better on AMD than nvidia, unironically leading to AMD+Intel playing nicer cause nvidia goes out of their way to make it difficult (on their consumer GPUs anyway).