the devs like to pretend that it is and say how only a small part of it is proprietary, but it's a deceptive PR stunt because they're counting the whole chromium source into that statistic. if we don't consider the chromium source they didn't write, their own vivaldi code is largely proprietary.

regardless, a program consisting of 99.9% free code and 0.1% proprietary code is proprietary as a whole - you can't audit, modify or redistribute copies of it as long as any non-free code is present.