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Some of the best examples are things you might not even think of: keyboard shortcuts, right click menus, long presses, double clicks.

I find Vivaldi (not my daily browser) pretty good at that: by default it's a browser like any other, but you can personalise everything pretty much everything, and unlike something like KDE it feels thoughtful and easy to use

VSCode, Firefox, KDE, tilix, cmder, the latest version of blender...

I would agree with VSCode, seems ok in my book.

Firefox of the past maybe, when it allowed unsigned extensions. AFAIK now it isn't possible in non-dev version.

Others I have not heard of/not used.

In its early days Firefox had this. And there's no reason any of it would need to be dumbed down or locked down.