It seems that when exporting a model that is set to “Unlit”, the result neither respects the roughness that was set (e.g. roughness=1) nor respects the “Unlit” part and sets KHR_materials_unlit in the exported glTF.
Either of those would be great to have Not sure if that counts as bug (because roughness isn’t exported when model is set to Unlit) or feature request (for exporting KHR_materials_unlit), maybe both.