Unless Opera calls the built-in mod that it comes with "The end" and shows it at the bottom of the opera://mods page, that's not a mod and is just a little design showing the end of the mods page.
Maybe you're talking about the new UI in Opera GX. To revert to the old UI, you can goto the URL opera://settings/widgets_settings, enable "Widgets" and "Show new look", open a new tab, click "Widgets" at the top right of the start page to pull out the Widgets drawer and click the back/revert button to the right of the "Let us know" field.
Just keep in mind that Opera will probably take away the option to revert eventually.