I never used the mail/news/bittorrent clients much - but outside of these there's still some Opera features that I miss.
Maybe I'm crazy but I liked "ask me each time" when encountering new cookies - prompting a pop-up with site-level settings and i'd usually set primary: allow, or session-only, and third party:block or session-only. And I miss the wonderful right-click "Block content" option. So for all the sites I visited regularly settings were saved, but whenever I went somewhere new I could really customise things to my liking, and enable/disable what I wanted. None of the defaults in modern browsers allow me to do that so easily.
Having said that, I agree with the general sentiment of your post.
Vivaldi also manages things I didn't think were possible with the Chrome engine, such as tiling tabs, and brings new features we didn't have with old Opera (and usually executes them well too) such as screen capture.
So yes, as much as I'd have liked to see the Presto engine moving forward, I'm grateful the Vivaldi team are delivering such a good browser.
To the previous poster - what are you using then? Other than Vivaldi, I still use Firefox, but it's certainly not improving. I don't see Pale Moon as a viable alternative - new websites won't support it just as they won't support Opera 12.* . Edge? Chrome?