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 conventio

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.