Some sites of historical significance were intentionally destroyed, particularly if the memory is painful. Most notably it has been very hard to preserve what remains of the Berlin Wall as a memorial.
That's an interesting contrast with places that are important but not honored - I know that some concentration camps have been preserved as museums to the horror but the Berlin wall is kind of a great counter example because not only does the city not want the wall there anymore (obviously)... but Berlin is also specifically opposed to trying to preserve the path of the wall since it'd reinforce the division in the city.
There is, from the Berliners I've spoken too, a strong desire to make sure the memory of what the wall did isn't lost - but all but the most minute physical presences of the wall were actively removed.
Some sites of historical significance were intentionally destroyed, particularly if the memory is painful. Most notably it has been very hard to preserve what remains of the Berlin Wall as a memorial.