@fjuk We don't 'choose' to limit this functionality. Unfortunately there are complicated legal restrictions around distributing support for H264 video decoding, and it comes down to 'we can't give you this functionality ourselves'.
Therefore we rely on others to give it to you. On Windows and Mac, it's the operating system that provides libraries that do H264 decoding, and Opera uses those. On Linux it's a bit more complicated, but it comes down to 'we use what Chromium uses except we can't give it to you ourselves'.
Unfortunately this means that now and then, when the library Chromium uses changes, Opera needs a certain minimum version of the library to be able to work with it. The rule of thumb is that Opera can always use a library if it's the same version as the Chromium version Opera is based on (check opera://about to see which Chromium it's based on), and sometimes older versions, depending on when Chromium last changed the library. Since the library changed in Chromium 67 / Opera 54, we currently can't use older ones.
While we are waiting for the stable package to be updated in Ubuntu's repositories, if you want a more official, Ubuntu-supplied up-to-date version of the codecs package, there's a package available for Cosmic Beaver which is already at Chromium 67. You can find it here: https://packages.ubuntu.com/cosmic/chromium-codecs-ffmpeg-extra .