I have recently upgraded to 2.0 so just a hand full. I think I had the add on called conversation improvements when I had 1.5 - so if you leave a conversation it removes it from your inbox? And if that said person that you left the convo with responded at another time to that same conversation - would it again appear In your inbox?
"Delete" has never been an option in conversations. It has been "Leave" since before 1.0 was out. The behavior surrounding leaving is the same as in 1.x (including what happens with replies, which is dependent on what you choose when leaving).
I can't really speak to what behavior an add-on may have changed though.
Is delete not an option because there might be more than two people participating? I still think of it like clearing out an inbox, but I guess it doesn't work like that?