FMT used unsigned char as an internal type to represent single-byte characters. But it does not seem to matter and clang16 does not like the unsigned version:
duckdb/third_party/fmt/include/fmt/core.h:295:30: warning: 'char_traits<unsigned char>' is deprecated: char_traits<T> for T not equal to char, wchar_t, char8_t, char16_t or char32_t is non-standard and is provided for a temporary period. It will be removed in LLVM 18, so please migrate off of it. [-Wdeprecated-declarations]

From the spec:
https://en.cppreference.com/w/cpp/language/types

char - type for character representation which can be most efficiently processed on the target system (has the same representation and alignment as either signed char or unsigned char, but is always a distinct type). Multibyte characters strings use this type to represent code units. For every value of type unsigned char in range [0, 255], converting the value to char and then back to unsigned char produces the original value. (since C++11) The signedness of char depends on the compiler and the target platform: the defaults for ARM and PowerPC are typically unsigned, the defaults for x86 and x64 are typically signed.