Today I updated to Opera 68. The "Recently closed tabs" button was replaced by the "Search tabs" button, I was ok with that, since I can see the recently closed tabs in Menu > History.
My big problem is that open tabs in my other devices won't appear anymore. There were a button in my sidebar, but now it's gone, I can't find it anywhere.
So the only workaround is to disable the flag suggested above
No "Open tabs in other devices" button, it was there a moment ago but now it's gone.