Can you provide a simple .blend file showing the problem?

I atached a blend file (ShaderToRGB_Problem.blend) here.

This file was saved by blender 2.83. (I checked that it can be read by 2.90 and the problem occurs there with my PC.)
When viewport render is turned to "Rendered", the suzanne becomes fully transparent in 2.90, while it is shown in 2.83.

During depth prepass the BSDF evaluation is skipped as at this point not all material resources are available. [See closure_lit_lib.glsl].

#if defined(DEPTH_SHADER) || defined(WORLD_BACKGROUND)
  /* This makes shader resources become unused and avoid issues with samplers. (see T59747) */
  return;
#else

This will output black color, what is converted and put in the alpha socket. An alpha of 0 is not visible, making the depth shader always discard the pixel.
As a work-around you could use the alpha blend mode, what does not rely on the depth shader.

There isn't a good solution for this as the depth shader must run first before anything as other effects (AO, shadowing) rely on the depth buffers. Surface shading happens later in the pipeline as they rely on AO and shadowing. For now I set it to known issue and add this as a limitation in the manual.