If I say I do not remember it may mean that I refuse to discuss that memory. No one has a delete feature in their brain, at least none that I can access purposely.
Of course everyone remembers to some extent the past, perhaps not in the form it happened, but in the way the brain interpreted it.
To say I do not remember is the same as saying I never knew, or simply, I do not remember. If you cannot read thoughts there will be no way for you to be sure I actually remember but refuse to acknowledge the memory.
With adequate care one might convince others that it doesn’t matter whether you remember or not, that issue, subject, is closed and cannot and will not be discussed.
People change over the years, sometimes for the good, sometimes not. If they treat you shabby now whereas in the past they did not, then why not selectively “forget” shared memories. They may protest and become angry when avenues that were once opened are found closed. Better that than to continue to allow them to drain you emotionally every time you meet. Some try to manipulate you through shared memories. If they cannot access them, then that means is closed.
How else to reject those who have become abrasive and now negatively affect your life?