Short description of error
There doesn't seem to be a way (anymore) to make an object appear shadeless in solid view. With Object, we only have color settings, and with Material we only have color, metallic and roughness, but no parameters to make an object ignore workspace lighting altogether. Setting color to Texture in solid mode is useful for reference images. But currently, they would either be affected by the viewport specular lighting, or, if that's turned off (which is undesirable), would still be affected by shading based on camera view angle.