Unicode 15 released

Meanwhile, Windows is never going to give up UTF-16, unless they have a full backcompat break. The problem, you see, looks roughly like this:

0. In the bad old days of everyone using a different ISO-8859 variant (except for East Asia, where they had a wildly different set of encodings because CJK scripts are huge), Windows came in different editions, and the code page was baked into the OS. All of their APIs would transparently use the OS-level code page, and if you wanted to support other code pages, too bad, it was impossible.
1. At some point, they decided that was too ugly, and modularized things to such an extent that you could install multiple locales on the same computer and switch between them. But the old APIs were still around, so the OS-level code page became the application code page, set by default to "whatever the active locale specifies," and the API grew a few functions for changing the code page if desired. Eventually, they also added manifest support so you could just do that at the packaging stage instead of having to write actual code for it.
2. The Unicode Consortium comes along and tells everyone "We're doing this great new encoding, it'll have all the languages and fit into 16 bits." Microsoft decides to go all-in on this, and deploys a brand-new set of APIs which are identical to the old APIs, except they use wchar instead of char, and only accept UTF-16 (native endianness, usually LE). The old char APIs are informally deprecated but continue to exist for backcompat reasons. Also, they introduce a whole bunch of preprocessor macros so that you can just code against the two APIs as if they were one API, not think about charsets at all, and then decide which API to use with a single global #define at build time.
3. Everyone figures out that 16 bits is not enough, and surrogate pairs are born. Microsoft bolts on surrogate pair checking to their existing UTF-16 APIs and calls it a day. (Technically, before surrogate pairs existed, it was called "UCS-2" rather than UTF-16, but I have avoided using that name to prevent confusion. It's exactly the same encoding, other than the existence of surrogate pairs.)
4. Everyone figures out that UTF-8 is far superior to UTF-16. After much hemming and hawing, Microsoft adds a code page for UTF-8, as if it's just another legacy encoding, but eventually they update their documentation to vaguely suggest that maybe using the char API with the UTF-8 code page is better in some circumstances. Also, they make the UTF-8 code page the system-level default in all locales, but that only affects the char API, because the wchar API never used code pages in the first place (it's hard-coded to UTF-16 and always has been).
5. So now, the wchar functions must continue to exist, and must continue to use UTF-16, or else lots of applications will stop working. Microsoft *could* tell everyone to recompile against the char functions with a UTF-8 code page, and then drop wchar support, breaking everyone who didn't recompile, but they are not Apple and can't get away with doing something like that. Also, the wording of their documentation strongly suggests that much of the NT codebase uses UTF-16 internally and that it's the "native" encoding of modern Windows, so changing the APIs would be putting lipstick on a pig anyway.

They still often do, even today in Android's YouTube app I'm seeing it truncate "collapsed" descriptions at length X, splitting an emoji in half, and showing a � at the end...

> if you try writing a document with a 5-byte code point, many UTF-8-aware applications will throw an exception or replace it with U+FFFD, so you'll have to update every application before you can start using the new encoding (i.e. it will never happen).

Yeah, throwing an exception for this would be really annoying; it's what really ticks me off about the entire situation – not that we'll need an extension anytime soon, but still, we *had* this capability and we deliberately removed it. It's like 240.0.0.0/4 all over again.

On the other hand, showing a 5-byte codepoint as U+FFFD isn't too different from showing a surrogate-pair-encoded character as two U+FFFDs that UCS-2 software used to do.