If there are no placeholder settings defined then we cannot add any components to the page through Page/Experience Editor in any version of Sitecore?
That is correct. Experience Editor works for adding components by having somewhere to add the component (the placeholder) and the options are dictated by the placeholder settings and the security applicable to the current editor.
If you are building for Experience Editor support, you will need to have placeholders to have visual adding to the page using the "add component" button. Users can add renderings via the presentation details dialog manually, but this is not a best practice and should be avoided.