Yes, I know they debated. I was thinking of Nixon’s subsequent successful runs. I do not remember Nixon debating either Humphrey or McGovern.
I was living in West Berlin when Nixon ran against Humphrey, so perhaps I didn’t see it. I am pretty sure he didn’t debate McGovern.
There was a hiatus in the debates after 1960 until 1976. Since then, there have been Presiential debates every election.