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Brave disables or removes a number of Chromium features: https://github.com/brave/brave-browser/wiki/Deviations-from-...

Vivaldi does quite a bit of upstream development (for their team size), and it is as close to the original Opera as you're going to get. It is proprietary, though (but their history w.r.t to treating their users right is better than everybody else, except maybe Apple).

Some offer functionality not found in the main browser they fork from, like Waterefox having the ability to share normal, private and tor tabs all in one browser window.