What is the ruling on saying "I swear by triple divorce that I did not know until now" during a dispute, given that the speaker does not mention his knowledge of the thing that the other party claims to have informed him of previously?

As long as you swore, believing yourself to be truthful in what you swore about, and you did not intentionally lie, then the preponderant opinion is that you have not broken your oath. Therefore, no divorce or expiation is binding upon you, even if the reality is contrary to what you swore about. It should be noted that the permissible oath is by Allah Almighty, whereas swearing by divorce is among the oaths of transgressors, and it may lead to undesirable consequences. So, beware of swearing by divorce.