Note that you can add them yourself by going to the URL opera://flags/#sidebar-site-panel, enabling the flag, restarting Opera, clicking the 3 dots at the bottom of the sidebar, scrolling down to "Custom site panels" and add the URL for the service.
Just note that custom site panels are strict. They don't allow redirects to another domain. So, if you click a link to another domain, that link will open in a new tab instead of navigating to it in the panel. Likewise, if the mail service you add redirects to a different domain for logging in, you'll need to log in via a regular tab first to set the session cookie and then refresh the panel.