The IPC is still GCN even if the microarchitecture is RDNA. Just like the IPC on CPUs are x86/64 with different architectures, and nVidia has been also been using the same IPC, with different architectures. All for compatibility.
While that may be true that doesn't mean you can go bare metal and expect it to work on any GPU model other than the one you develop it for. It
might work. That's about as much as you can say. Not only that but it'll never work on a GPU of a different architecture. That's why you need hardware abstraction. It basically makes it a non-factor, at a small cost to performance. The PS4 and Xbox One were built with backwards compatibility in mind. They could slap an NVIDIA GPU in the PS5. with the appropriate drivers of course, and PS4 games would still work.
They are not used in PC world
Of course they are. For example, for months Sonic Ether's Path-traced GI shaders worked on NVIDIA but not AMD cards. Do you know why? Because NVIDIA provides an extension to the OpenGL API (NV_geometry_shader4 specifically) that lets geometry shaders accept quads, or perhaps more accurately automatically tessellates them into tris, and AMD doesn't. On AMD cards geometry shaders only support tris. It does work now because SE found a way around it, if I recall correctly he axed the geometry shaders.
This idea that you have that extensions aren't used on PC...it's just demonstrably wrong. Of course specific extensions are going to be different between the platforms but that doesn't change the fact that extensions exist on PC.