I confirm OPERA_AUTOUPDATE_DISABLED can't stop auto-updates from happening by itself anymore.
If you really want to stop the auto-updates try this:
Set the OPERA_AUTOUPDATE_DISABLED environment variable, you still need it to prevent Opera attempt to auto-update when you run it.
Search for the Windows' task scheduler, it's a Control panel item.
You should see a list where one of the items is "Opera scheduled Autoupdate (...)", right-click it and select disable.
But please don't do that if you aren't dependent on a feature that has been changed or removed in the latest versions. If you absolutely must do this, don't forget to check info about new versions once in a while, they contain security updates and will give the best compatibility with websites, the system, etc.