Elsewhere in the thread is an example of a system setting the top bit when storing ASCII in 8-bit files. But storing it as an 8-bit encoding is also a convention rather than a requirement, and any file with an alternative encoding would not be valid UTF-8.
Anything with a defined 8th bit would be an extension of ASCII.
But it was obvious back then that if you use 8 bits for a character that the 8th bit would have to be unset unless some further processing was happening.
Even more so in the context of UTF-8. 8 bits bytes had won by then and in that environment ASCII was a thorough sub set of UTF-8.