I think you're confusing text documents like PDF or word .doc(x) type files for "plain text". Plain text doesn't contain hidden anything at all. It's just plain text and nothing else. That's why a plain text editor is completely different from Microsoft Word, or OpenOffice. They both edit textual document types, but a plain text editor doesn't do metadata, fancy fonts, colors, styles, or any of the fun junk you find in PDF and other textual document types. It does only text.
"Plain" text can contain hidden characters such as zero width spaces which won't be visible to someone reading it but could be used for identification purposes.
I know what "plain text" is and even so, it can contain identifiable markers, if you want to track down a leak let's say.
"Something 𝗌omething will launched on Novemᖯer 3rd"
This "plain text" string has two markers that aren't actually ascii, `𝗌` and `ᖯ`. By changing the markers for each person you send it, you might be able to track down who is leaking things, if they're careless and just copy-paste stuff that look like plaintext but actually isn't.
