"invisibly formatted text elements that "survive" even being copy-pasted into a plain text editor" Plain text is unformated, so that is not possible to my know

"invisibly formatted text elements that "survive" even being copy-pasted into a plain text editor" Plain text is unformated, so that is not possible to my knowledge. However, there are some invisible unicode characters https://invisible-characters.com/ https://invisible-characters.com/ Whith them you might be able to encode hidden data into plain text, but you would have to decode it, to have text again.