Opera has an experimental way to add sites to the sidebar. Goto the URL opera://flags/#sidebar-site-panel, enable the flag, restart Opera, click the 3 dots at the bottom of the sidebar, scroll down to "Custom Site Panels" and add the URL for the page you want.
Just note that site panels are strict. Redirections to difference domains are blocked and clicking on a link to a different domain will open the link in a tab instead. Because of this, for some sites like Outlook.com that redirects you across domains when logging in, you'll need to log in via a tab first and then reload the site panel.
Also note that you can't adjust the icon for the site in the sidebar. You also can't control the position of the icon in the sidebar.
But, it's something.
