> Most GUI toolkits do not.

Why are you guys talking like there were dozens of GUI toolkits in mainstream use? It's basically web stuff, Qt, and then everything else. Web would be UTF-16 as discussed above, Qt is UTF-16, and even if we entertain the admittedly "large just behind-the-scenes" Java/.NET market, that's also all UTF-16. WxWidgets being a fence sitter can do both UTF-8 and UTF-16, depending on the platform.

Which players am I missing? GTK and ImGUI? I don't think they are too big a slices of this pie, certainly not big enough to invalidate the claim.

Anything that is using the C stdlib at one point.