What does that even mean? We know exactly how the Saturn V worked, but we have no production lines to build any more, nor would we want to build a big rocket that way today (see above). Technology has evolved past a rocket that was designed in the late 1950's. You know, the Saturn V "instrumentation unit" cost close to a million dollars and had less computing power than the computer on your desk? Why would we want to duplicate that?
We know all about navigating spacecraft through deep space (how do you think we get those landers to Mars?).
We know all about keeping people alive in space for periods much longer than it takes to get to the moon. That's what ISS is for.
So what "knowledge" have we lost, exactly? We don't have working hardware or the production lines to build them. If there were a national emergency requiring us to land on the moon asap, we could be back there in five years, tops. That it takes longer than that is the result of bureaucracy, lack of funding, and lack of urgency.
all of the documentation of the tests and results are gone. scrapped. Junk heap of history.
Men risked their lives to plant a flag and were stabbed in the back. Moonwalkers feel betrayed that in for 40 years manned flight exploration was killed off.
There is a site that refers to it as “the second assassination of JFK”.
Not saying we are going to “rebuild it” but plenty of lessons were learned from successes and failures.
The point is even if we wanted to WE CANNOT REBUILD IT. We lost the technology designs.