Even if your editor "supports" other encodings than ASCII -- i e. it can save text in other formats -- many (most?) of them can be set to save in some particula

Even if your editor "supports" other encodings than ASCII -- i e. it can save text in other formats -- many (most?) of them can be set to save in some particular encoding by default (so set it to save as ASCII), and usually they save previously-existing files in the same encoding the files already had. And, as someone already mentioned, all the first 127 characters of the UTF-8 encoding are the same as the ASCII ones, so a UTF-8-encoded file containing only those characters is identical to -- is -- an ASCII-encoded file.