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Chat input is typically single-line, submit-on-enter. Messaging apps seem to be transitioning to optional multiline but the default action for enter still seems to be submit so it's perfectly reasonable to use <input> for chat input.

Well, inline chat, like IRC is generally a single-line input... though I suppose you could use a textarea rows="1" and strip \n on paste, etc, that wouldn't be too bad as you're likely listening for \n to post the message anyway.

Either way, not using a typical name/id for the text input would probably work better in terms of not getting suggestions...

That would indeed be the norm for forums. But not for chat. Real time chat is typically more usually a single line entry. Having auto-complete on that is irritating and utterly spoils the UI.