8-bit clean is an attribute of computer systems, communication channels, and other devices and software, that process 8-bit character encodings without treating any byte as an in-band control code.
7-bit systems and data links are unable to directly handle more complex character codes which are commonplace in non-English-speaking countries with larger alphabets.
But some implementations really did not care about formal discouraging of 8-bit data and allowed high bit set bytes to pass through.
RFC 3977[7] specifies that "NNTP operates over any reliable bi-directional 8-bit-wide data stream channel", and changes the character set for commands to UTF-8.
The Internet community generally adds features by extension, allowing communication in both directions between upgraded machines and not-yet-upgraded machines, rather than declaring formerly standards-compliant legacy software to be "broken" and insisting that all software worldwide be upgraded to the latest standard.