Neither Sony or Microsoft publish standards defining what features require a game to have a paid subscription to function with online capabilities. It used to be pretty clearcut, but the rise of F2P games muddied the waters forever, with many of them no longer requiring a sub, the assumption being that Sony/MS are taking a cut of the cash shop instead.
Long way of saying I don’t think it’s possible to prove what I said, but it’s a logical conclusion based upon what we know. Name one game, on either console, that has text based chat, and doesn’t require the premium sub. I can’t think of one. That seems to be their new red-line: Voice chat in some cases is OK, but if there’s text, it requires the sub.
FWIW, D4 as-is requires a PS+/XBL sub to access certain features, but they’re so minor you could play through the whole game without noticing:
https://twitter.com/pezradar/status/1649433665984757761
I think a text chat that didn’t work would be more noticeable.