RetroArch uses all of them. HLSL, GLSL, Vulkan, Slang, Cg as well as proprietary shader formats used on consoles, as some platforms only support specific formats, so they often convert between all of them to offer as many shaders as possible across all combinations of platforms.
But one of their big contributors also worked on Vulkan and Slang/SPIR-V itself if I recall correctly (and works at ARM on GPU drivers), so there's that.
Retroarch can ingest GLSL, Cg, and its own extension of GLSL it calls "Slang shaders." Cg is deprecated. It uses SPIRV-Cross, the de facto solution to transliterate to other APIs. The "slang" in this article is an entirely different "slang".
The guy who created Retroarch, Themaister, seems to work for Valve now, and continues to work on SPIRV-Cross among other things, making gaming on Linux actually viable.
But one of their big contributors also worked on Vulkan and Slang/SPIR-V itself if I recall correctly (and works at ARM on GPU drivers), so there's that.