It is straightforward for a reverse engineer to attach a graphics debugger to an OpenGL application to extract the shader source code. It is my understanding that Vulkan, on the other hand, uses SPIR-V bytecode, rather than passing plaintext shaders to the graphics API.
Does SPIR-V bytecode obfuscate the shader source, or is it fairly easy to decompile?
There is an entire specification explaining, in explicit detail, the behavior of each and every SPIR-V opcode. That's kinda the opposite of obfuscation. But there's more to it than that.
SPIR-V, despite being "assembly", retains a rich amount of information about the source program. It contains structure definitions, function definitions with parameter and return types, looping and conditional constructs, etc. Writing a decompiler for SPIR-V is not at all difficult.
SPIR-V also can optionally contain fragments of text that annotate various SPIR-V definitions. This is more of a function of the environment that compiled to SPIR-V, but the output SPIR-V can contain variable names, structure names, and etc. These OpName decorations can all be easily culled if you wish.
But even without names, all of the important structural information is there. So the security gains from SPIR-V compared to raw GLSL is rather minimal.