>What is stopping [...] Java, JS, and C# files in UTF-8?
The output of files on disk can be UTF-8. The continued use of UCS-2 (later revised to UTF16) is happening in the runtime because things like the Win32 API which C# uses is UCS-2. The internal raw memory of layout of strings in Win32 is UCS-2.
Code page 65001 has existed for a long time now, but it was discouraged because there were a lot of corner cases that didn't work. Did they finally get all the kinks out of it?
When Windows adopted Unicode, I think the only encoding available was UCS-2. They converted pretty quickly to UTF-16 though, and I think the same is true of everybody else who started with UCS-2. Unfortunately UTF-16 has its own set of hassles.
Yeah, there's sometimes a lot more hacks like WTF-8 and WTF-16 in practice on UCS-2 originally systems (including Windows and JS) than is healthy: https://simonsapin.github.io/wtf-8/
The output of files on disk can be UTF-8. The continued use of UCS-2 (later revised to UTF16) is happening in the runtime because things like the Win32 API which C# uses is UCS-2. The internal raw memory of layout of strings in Win32 is UCS-2.
*EDIT to add correction