There is a way to revert ui.
Go to C:\Users*USERNAME*\AppData\Roaming\Opera Software\Opera GX Stable\ and edit the "Local State" file with any text editor.
Using Ctrl+F find "gxx_flags":{"enabled":true," and change "true" to "false".
Sadly it work only for 1 session and reset for second.
P.s. I lost all my tabs, logins, passwords, settings and saves while seeking for it.
Whoever force new ui without possibility to revert should be fired.