About 20 years ago I used Cyrillic confusables to watermark internal documentation that was being leaked by a disgruntled customer service employee. The document would dynamically render and include the employee ID based encoded as bits in the text. It survived copy/paste to plain text well.

I did run into some issues in early versions on when characters in Linux commands or visible web addresses were replaced. Fortunately the source docs were HTML, and it was easy to exclude code or pre nodes when rendering.

I thought this was so clever, but the leaker was never caught using it, to the best of my knowledge.