Even if all wire format encoding is utf8, you wouldn't be able to decode these new high codepoints into systems that are semantically utf16. Which is Java and JS at least, hardly "obsolete" targets to worry about.

And even Swift is designed so the strings can be utf8 or utf16 for cheap objc interop reasons.

Discarding compatibility with 2 of the top ~5 most widely used languages kind of reflects how disconnected the author of this is from the technical realities if any fixed utf8 was feasible outside of the most toy use cases.