I had faced the same problem recently as opera://plugins is no longer working. If you want to check/update your flash plugin version, you may use the link: https://helpx.adobe.com/flash-player.html
You will be able to check your flash version and if needed update it.
That page does not give the whole version number. The whole number is 25.0.0.171, and I want to see the entire number. Now, I need to locate the plugin on my hard drive, open the app package and read info.plist just to read the version number. I should not have to go to extremes like that just to read a version number.
I can read the whole version number from settings in Safari and in Mozilla browsers, so why not with Opera?