Nope, Opera notifications are dead (Stable/GX/Legacy/x86/x64 versions under Win7/Win10).
You may re-activate this function for once, and it will work during the current session. But after you launch Opera for the next time, pop-out notifications won't work.
Well, that's lame. I really liked GX but I'm not living without notifications.
In the Site Settings screen, scroll down and select Notifications. The following two settings are offered by toggling the switch on and off: Ask first: Default option. You need permission to send push notifications to a site.
Lol. This isn't my first time using a computer bro.