Hello vladyslav.volovyk, I would be willing to pay up to $100 or something like $4/month to have this addon actively developed. I am sure plenty others would too. I suggest that you calculate the possible income from such a thing. Please consider it. Probably the most promising extension ever developed for web browsing since the birth of web browsers. Abandoned. What a shame. 

If anyone were to develop a similar extension (and some have), please make the interaction instant, like tabs outliner. It is crucial that the tabs are saved as links/html bookmarks and not suspended tabs. It is also imperative that tabs do not take lots of vertical space. These are lines of text, for crying out loud. We don't need these big spacious cards stuck in under each one. Being able to see 50 or 60 tabs in a list without vertical spacing has a drastic effect on the possibilities in browsing. 

And of course, ideally, with the ability to select multiple tabs at once.

I have tried the current alternatives to tabsoutliner, and none of them come close to delivering like it does. 

While TO currently works in the sidebar in Vivaldi, it does not open a proper window like in chrome/edge (it's far too wide, with the minimum width=600px). The enormous benefit of getting that to work would be keeping the vivaldi panel open and available for other things such as bookmarks on the opposite side of the browser. Would really really love if TO could open in a proper narrow window to enable this functionality.