Most of them were told "Never, ever speak about any of this".
And they didn't.
Like the Zanryu Nipponhei [0], they were loyal to the last. Even my
own father kept things about his airforce days way too tightly wrapped
up long, long after the official secrets sell-by date. I have some
admiration for this, but in the end it's a loss to historical record.
Consider it a blessing. Those orders were about controlling history much more than 'secrets': Many things done in war are later considered war crimes. Your admiration might have taken a hit had he started talking gleefully about the time he - just to pick a random example of things that happened - heroically shot fleeing "enemy" children on the ground.
And they didn't.
Like the Zanryu Nipponhei [0], they were loyal to the last. Even my own father kept things about his airforce days way too tightly wrapped up long, long after the official secrets sell-by date. I have some admiration for this, but in the end it's a loss to historical record.
[0] https://en.wikipedia.org/wiki/Japanese_holdout