It's completely and utterly irrelevant that Chromium is open source, because the web is a protocol, and having the source for an implementation of the protocol doesn't matter in the least when you don't control the protocol. You can't just fork Chromium and remove a feature, because websites expect the feature, and your browser won't work on them. You can't just fork Chromium and add a feature, because websites don't care about your tiny fork and won't use your feature. You can't fork Chromium, you have to fork the entire web.
Of course you can. Microsoft's Edge and Brave already add proprietary features like AI and reader mode, tab groups, video calling, crypto wallet etc.
Brave could add a custom CSS or HTML feature. Hell that was the status quo we came from ten years ago when each vendor had their own feature flags and implementation for WebRTC and proprietary video codecs, etc.
Brave already explicitly removes ads and blocks all kinds of things websites expect to work on Chrome.
Nowadays Edge has some superfluous features that differentiate it from Chrome, but they are still superfluous. Underneath it's still Chrome, because the internet demands Chrome.