This is really a subjective question so answers will vary. The best I can give you is some statistics about graduation rate and let you draw your own conclusion.

In 1870 2% of the US population had the equivalent of a high school education.

By WW1 about 15% of all children graduated from high school.

In 1940 the graduation rate topped 50% for the first time.

In 1960 the graduation rate peaked at 77%

By 1970 the graduation rate had dropped below 70% again and would continue to stagnate through the 70's and 80's.

In 1990 the graduation rate had again risen to about 75% before beginning another decline through the early 90's.

Government publication with educational statistics

My personal opinion is that a high school diploma ceased to look impressive in the early 60's when the percentage of citizens in the United States with a high school diploma topped 50%. Of course answers will vary depending on your definition of impressive and on where the resume is going. I know of places in the US today that consider a high school diploma impressive.

I know it was not the original question, but I can add some perspective.

In Sweden, the equilent is studentexamen which was a test you needed to pass from 1867 to 1968, when it was replaced by simply requiring the student to have high enough grades to not fail any parts.

1900, about 1% of all youngsters studied and passed the tests.

1950, about 5% passed.

1960, about 10%.

1970, about 30%.

1980 it peaked at about 85%.

2000 and today the number is about 80%.