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.
I was thinking of DEL, but was obviously mistaken. Thanks for catching that!
The "char" type in D represents a UTF-8 code unit, the byte 0xFF is not a valid character code and is strictly forbidden.