Reference letters should be submitted with formal letterhead, which usually includes physical address, phone numbers and email addresses. That's how my references sent in their letters (NIW application).
It's very unlikely that USCIS will contact directly your references, given their backlog and the amount of time they dedicate to each case.
However - everything you submit to USCIS must be genuine. Any publication/media item/reference letter/statement must be true. Failing to do so may cost you your GC (even years after you obtain it) and might come back to bite you in the posterior even after you naturalize to become a U.S. citizen.
