Looks like, on Ubuntu 20.04 the fix is as simple as installing the latest version of chromium-codecs-ffmpeg-extra from http://security.ubuntu.com/ubuntu/pool/universe/c/chromium-browser/ for Chromium your Opera version is based on. I just installed chromium-codecs-ffmpeg-extra_97.0.4692.71-0ubuntu0.18.04.1_amd64.deb and video playback in Opera 83 works again. No need to symlink libraries.