@paultatarenko I submitted an update of Classic Tabs to Opera's addons site about ten months ago, and for whatever reason they won't publish it or even respond to me asking why it hasn't been published yet. I've long since given up on it ever getting published.
If you want to try the latest version, you can download it from here: https://github.com/ChaosinaCan/ClassicTabs/releases/tag/v2.0.1-rc.0
More info on how to install it can be found in this conversation: https://github.com/ChaosinaCan/ClassicTabs/issues/33
I do not plan to keep maintaining the extension, as Opera won't publish my updates and I mostly use Edge now. If anyone else wants to maintain it, feel free to fork it on GitHub.