My examiners didn’t ask me what I was going to do after my PhD, but I know candidates who were asked in theirs. Had I been asked, I probably would have been stumped by the question. No idea. The thoughts that lead me on to my current path came months later. It might have been useful to have some ideas that I could have shared in the viva, had it come up.
(the only ideas I had about future plans were related to how I might develop my thesis research, what directions or improvements could be made with more time, different resources, etc)
Being asked about plans can be scary, but remember: it doesn’t have to be final.
A plan doesn’t have to commit you. A plan is potential. A plan is a sequence that you could follow, not one that you are following or even one that you will. You don’t have to have the best plan. You don’t have to have all of the ideas worked out. You just need something.
So: what’s your something? If you don’t know, maybe a better question to think about and answer is “Post-viva, what do I want?”