'What was it about?' is the more normal phrase. This can refer to the subject of a conversation, a story, a film - almost anything. You wouldn't use 'on' in these cases. 'on' is used more in studying and business, to refer to the topic of a reference book, a lesson, a lecture, a seminar, a conference, or similar. So if your colleague at work went to a seminar, you might say 'What was it on?' ie what was the topic? If you see a classmate studying an academic book, you might say 'What's it on?'. But you could also say 'What's it about?' in these two cases. Or in other words, if you are not sure, always use 'about', because it's a more general question.

Su. Ki is correct. Another way of completing the thought; "What was it on?" would be to understand the thought this way: "What subject was it on?" Native English speakers have managed, over time, to occasionally "jumble" their meanings and expressions, so that they occasionally use expressions like "What was it on?". It does created a slight confusion. I have learned to avoid repeating such confused expressions. I would write the inquiry like this. What was the event about? What was the activity about? What was the subject of the event? What was the purpose of the event? .