broken emojis? There apparently are known issues that some frameworks break Unicode at wrong boundaries, maybe the author saw it regularize into a deeper mess

It’s not just broken emoji, it’s straight up broken content: UTF-8 can not represent unpaired surrogates.

WTF-8 is necessary for Rust’s compatibility with Windows filesystems (it underlines OsString on Windows) as e.g. file names are sequences of UTF-16 code units (and thus may contain unpaired surrogates).