Since POSIX documentation allow it as an extension, there's nothing prevent implementation from that behavior.
A simple check (ran in zsh):
$ for shell in /bin/*sh 'busybox sh'; do printf '[%s]\n' $shell $=shell -c 'á() { :; }' done [/bin/ash] /bin/ash: 1: Syntax error: Bad function name [/bin/bash] [/bin/dash] /bin/dash: 1: Syntax error: Bad function name [/bin/ksh] [/bin/lksh] [/bin/mksh] [/bin/pdksh] [/bin/posh] /bin/posh: á: invalid function name [/bin/yash] [/bin/zsh] [busybox sh] sh: syntax error: bad function name show that bash, zsh, yash, ksh93 (which ksh linked to in my system), pdksh and its derivation allow multi-bytes characters as function name.
yash is designed to support multibyte characters from the beginning, so there's no surprise it worked.
The other documentation you can refer is ksh93:
A blank is a tab or a space. An identifier is a sequence of letters, digits, or underscores starting with a letter or underscore. Identifiers are used as components of variable names. A vname is a sequence of one or more identifiers separated by a . and optionally preceded by a .. Vnames are used as function and variable names. A word is a sequence of characters from the character set defined by the current locale, excluding non-quoted metacharacters.
So setting to C locale:
$ export LC_ALL=C $ á() { echo 1; } ksh: á: invalid function name make it failed.