I've also been searching for a way to do this. It feels odd that such a common feature has been missing for so long- seems it's become a niche requirement
Usually I can work around an application not having the option easily accessible by using my window manager, but Opera's window isn't recognized as a native window so that's not an option for me either.
This is one of those settings that you don't miss until you need it, and incredibly frustrating if it's not there once you do haha.
Opera, in my experience, is far superior in overall experience and features when compared against the competitors, so I'm not moving to another browser because of this, but I'll definitely add my vote for this feature.
I'm sure there are dozens more who would agree