So, Delta voted down their TA. Does that mean that anything that wasn't "on the table" and still being considered when the first TA was sent out can't be put back on?

RTTT certainly was on the table originally. The NC let it get taken off the table (actually they took it off themselves). "OTP made it too hard - we give up".

A "No" vote certainly could have forced them to put it back on. Just like anything else that was found lacking in the TA. That's the whole point of membership ratification - if it's voted down, you fix it. Obviously that ship has sailed.