> Even if the Unicode codespace were to ever be extended again (it won't), the only encoding that would become incompatible is UTF-16. In fact, both UTF-8 and UTF-32 are trivially extensible and used to be wider encodings, but were restricted to 0x10FFFF arbitrarily to match UTF-16 limitations.

Mind you, it wouldn’t be this easy, because things should perform Unicode validation, and many do, so every piece of software would need to be updated for the new, enlarged version of Unicode, UTF-8 and UTF-32, and old software that validated would baulk or convert anything from the new ranges into REPLACEMENT CHARACTER.

But yeah, UTF-16 would be toast.