So, this means all java based websites will be unreachable by Opera?
Nope, but their java applet will no longer work in Opera, Chrome, Vivaldi and all Chromium based browsers with current versions of Chromium.
Also, Java plugin is being discontinued By Oracle. It means that at a certain point until the end of this year, Java will probably stop working on all browsers.
IMHO, sooner or later plugins will be part of the past.