[1vuio0pswjnm7]

https://en.wikipedia.org/wiki/ASCII_ribbon_campaign

[1vuio0pswjnm7]

ASCII is always plaintext.

But plaintext may not always be ASCII.

Campaign started in 1998. How common was non-ASCII plaintext in 1998.

[layer8]

Except that “plain text” is more general than ASCII.

[zzo38computer]

Yes, although often ASCII will be suitable. (Sometimes, other character sets are helpful, though.)