Those types should not be optional. CHAR_BIT needs to be 8. It is clearly possible to implement the types even on a 6502 or Alpha. From the early days of pre-ANSI C, the language supported types for which the hardware did not have appropriate word sizes. There was a 32-bit long on the 16-bit PDP-11 hardware.
I would go beyond that, requiring all sizes that are a multiple of 8 bits from 8-bit through 512-bit. This better supports cryptographic keys and vector registers.
I would go beyond that, requiring all sizes that are a multiple of 8 bits from 8-bit through 512-bit. This better supports cryptographic keys and vector registers.