According to Vivaldi developers there needs to be a manifest.json file for Chromium 79+. I tried to use that manifest file for Opera but I couldn't make it work.
If you want to try yourself here that file: (Make sure you have right version of Widevine)
{
"arch": "x64",
"description": "Widevine Content Decryption Module",
"manifest_version": 2,
"name": "WidevineCdm",
"os": "linux",
"version": "4.10.1582.1",
"x-cdm-codecs": "vp8,vp9.0,avc1,av01",
"x-cdm-host-versions": "10",
"x-cdm-interface-versions": "10",
"x-cdm-module-versions": "4",
"x-cdm-persistent-license-support": false
}