Even plain text can contain hidden metadata.
"Something 饾棇omething will launched on Novem釚痚r 3rd"
This "plain text" string has two markers that aren't actually ascii, `饾棇` and `釚痐. 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.
Anything that is a homoglyph (https://en.wikipedia.org/wiki/Homoglyph) can be used in this manner