For now, 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 https://web.snapchat.com/. You'll then get a new icon added to the sidebar for Snapchat.
If you have lots of things in the sidebar, you might not see the icon until you scroll the sidebar down to the bottom. (There's no scrollbar for the sidebar, but it will scroll if there are too many icons to show all at once.)