Same as LUKE-LAMOTHE.
Unity 2019.4.8F1 (LTS):
FrameBufferFetch with COLOR0 as vertex input & varying.
Seems the varying COLOR0 conflicts with FBF's inout COLOR0

Just got this issue writing a very simple shader on a Mac.

struct vertexOutput {
float4 pos : SV_POSITION;
float4 col : COLOR;
};

this seems to be the cause of the issue, the COLOR declaration is for some reason causing the GLSL compiler to fail

Anyone found a workaround for this?

It appears that using COLOR0 when trying to build a GLES3 shader using framebufferfetch will not compile the shader (error given in Editor console log post-build when targeting GLES3):

Shader error in 'TheShaderName': Duplicated input semantics can't change type, size, or layout ('COLOR'). at line 163 (on gles3)

Compiling Vertex program
Platform defines: UNITY_NO_DXT5nm UNITY_NO_RGBM UNITY_NO_SCREENSPACE_SHADOWS UNITY_NO_LINEAR_COLORSPACE UNITY_ENABLE_REFLECTION_BUFFERS UNITY_FRAMEBUFFER_FETCH_AVAILABLE UNITY_PBS_USE_BRDF2 SHADER_API_MOBILE

And if you move your value over to COLOR1, your iOS project will build fine with no errors shown in Unity, but when the shaders are loaded on iOS, you will see:

Note: Creation of internal variant of shader 'TheShaderName' failed.
-------- GLSL link error: ERROR: Input of fragment shader 'vs_COLOR0' not written by vertex shader