The shaders are identical to the ones used in Tonga and Polaris, being of GFX8 generation. The Fiji cards have an older display engine (it's not capable of HDMI 2.0), and an earlier generation geometry processor compared to what's in Polaris. They also cannot encode HEVC. Otherwise, the cards are pretty much binary compatible as far as compute stuff goes. This review of the RX 590 on AnandTech explains it all in far more detail than I could ever reasonably put in a forum post, and I highly recommend everyone read it.
www.anandtech.com
Long before I joined, I followed your struggles with the R9 Fury lurking the TPU forums. Man, I know your pain. AMD hasn't been exactly kind to flagship buyers until the current generation, all of them having their bizarre share of issues... R9 Fury guys probably just didn't have it as bad as the Vega Frontier Edition buyers. I had them all.
No future updates means that the card's WDDM model and shader model will no longer be updated, and they will cease to receive new API features, even if compatible with the hardware. This gap will only grow larger and larger as 21.5.2 ages against more current releases. However, Windows 10/11 is fully backwards compatible with all WDDM models back to Vista's 1.0 - the card will continue to function for the foreseeable future, until Microsoft deems that earlier display driver model revisions are either unsafe or unsuitable to the operating system's demands.
For example, on the NVIDIA side, the final Windows 10 release of the Tesla generation GPUs (GTX 8000 through 200 series, 342.01) are actually Windows 8.1 model 1.3 drivers, this is for both GeForce and Quadro. They will still work, and I sometimes use such an old graphics card for some retro gaming on my CRTs.