I believe it’s worth reflecting on this a little before the viva.
What have you forgotten?
Perhaps you can’t know for sure. You’ve had to edit out papers, ideas and references that didn’t fit. You forgot them, to concentrate on what mattered.
Perhaps there are details that elude you sometimes; if so, what can you do make them more memorable or to summarise them?
Perhaps you view the question as an almost-irrelevance, a nonsense. How could you remember what you’ve forgotten?
I think it’s useful to remember that however much you have forgotten, accidentally or so you could focus, there must be so much more that you remember by the end of your PhD. You have a lot of knowledge, which doesn’t mean simply knowing more: you know more of what you need.
What have you forgotten? What do you remember? What do you know?