Feel free! The easiest way to do this is white text on a white background on the last page of the document. If you want to do it on every page do it in the header or the footer. Font size '1' for bonus points ;)

remember about aria-hidden or equivalent, otherwise you are making the text visible to screen reader users.

I have read many snide comments and personal ID numbers that way, mostly in public and public-ish documents.

Oh, that's funny. I don't mind people with screen readers reading it. The Venn diagram with 'people with screen readers' on one side and 'people that are about to break an embargo' must be an interesting subset.

Amusingly Nathan For You did this in a humourous way back in 2014 (S02E06 - "Party Planner").

What's the benefit? It appears that the outlook rejection was unintended. So against llm processing?

The embargo was enforced. This slightly mitigates scenarios where someone accidentally sends a document via email while it is under embargo.

Now I'm curious: what was in the watermark that would trigger Outlook? (I'm having some fun speculating. My guess right now is a ye olde "bu¥ vi@gr@ ch33p!!!!" spam trigger.)

No, it was a bit more subtle than that but I'm definitely not going to post the secret sauce in an open forum. The trick is to trigger the filters in a way that they'll block the mail completely, not just spam trap it.

Joining in on the speculation game: the EICAR string maybe?