Some alternatives for now:
You can goto the URL opera://downloads, right-click the tab and pin it so that it's pinned as a small tab to the left side of the tab bar.
You can goto the URL opera://downloads and click the heart icon to bookmark the page as a speed dial on the start page or a bookmark on the bookmarks bar folder or in "other bookmarks". That way you can quickly get to your downloads. You can also use ctrl + j to open the downloads page.
You can enable the "Custom user sites for browser sidebar" flag at the URL opera://flags/#sidebar-site-panel, restart Opera, click the 3 dots at the bottom of the sidebar, scroll down to "Custom Site Panels" and add opera://downloads as a sidebar panel. Then, when you open that panel, increase its width. You can also click the pin icon at the top right of the panel when you want so that it stay open and expanded to the left of web pages. You can also goto the URL opera://settings/keyboardShortcuts and set a shortcut for the "Show sidebar" command to easily toggle the whole sidebar on and off. I like using F4 for it.
(Note though, there's already a download panel in the sidebar by default.)