> ... Unicode says that 0xFF is an invalid character.

Not so. You may be thinking of UTF-8 encoding. 0xff is DEL in Unicode.

DEL is unicode codepoint U+007F, which is the byte 0x7F in UTF-8, not 0xFF. Perhaps you were thinking of ÿ which is codepoint U+00FF, which encodes to the bytes 0xC3 0xBF in UTF-8.

The "char" type in D represents a UTF-8 code unit, the byte 0xFF is not a valid character code and is strictly forbidden.