Non breaking spaces and zero width spaces are not watermarks. They are used all the time in professional typesetting and it's actually a good thing that GPT models can do that now.

But of course they can be a tell if you write your work email like a professional type setter but to be fair there are lot of other tells too in GPT outputs.

PS: In fact there are even keyboards (but they are rare) for example for German E1 extension (https://de.wikipedia.org/wiki/E1_%28Tastaturbelegung%29) that can even type those characters. They are used to prevent unwanted line-breaks, for example between numbers and there units or to allow for hyphenation in long words.