It always was the wrong word anyway. Except for the very few handsets that really can transmit and receive simultaneously, mostly it is not duplex, it is simply "split".
Google dictionary. du·plex "(of a communications system, computer circuit, etc.) allowing the transmission of two signals simultaneously in opposite directions"
The key word is simultaneously.
Of course, but the decision to all it "duplex" was to mirror what most radios (and their manuals) call the function.
Personally I would re-structure the GUI to have a TX and RX frequency and drop the entire duplex section, make an TX frequency of 0 be "no transmit". That would be a large change though and might just be my own personal preference :-)
That preference would confuse like 90% of the users who are used to programming a receive frequency and offset.
Can't you simply change the text in the GUI and leave the the token (or whatever moniker the language you write in calls it) as "off"
Of course, but then we'd have None, Off, *and* NO_TX to explain to people :)
--Dan