I can type the newline character on the default keyboard in the default Messaging app - the emoji button changes to a newline button when I press the "shift" / up arrow button on the left hand side.
Strangely I do no see that same behavior in other apps like Vanadium, just in the default Messaging app.
Agreed that it would be nice to have some kind of toggle that changes the behavior from emoji/newline to newline only for that button. If that's possible, I'd love to hear about it.