Wouldn't it be rather the case that char8_t just wouldn't exist on that platform? At least that's the case with the uintN_t types, they are just not available

Wouldn't it be rather the case that char8_t just wouldn't exist on that platform? At least that's the case with the uintN_t types, they are just not available everywhere. If you want something that is always available you need to use uintN_least_t or uintN_fast_t.