@leocg Thank you. It still doesn't give the same functionality as Twitch sidebar extension in OperaGX, but at least it's something.
Speaking of which, I just now found out that OperaGX has already implemented Workspaces. So, I think I'll just switch to it and the problem will be solved for me.
But I honestly don't understand why not to add sidebar extensions from OperaGX to Opera, since Twitch and Discord are used not only by gamers.