There isn’t one reliable percentage that answers this question as written. We need to know which country you mean and what “able to read” means—for example, reading a simple sentence or understanding a high-school-level book.
There is another challenge: reading tests and graduation rules have changed since 1970. So comparing students from then with students in 2025 using different tests would not be fair.
In simple terms: graduating from high school means a student met that time’s graduation requirements, but it does not tell us exactly what percentage could read at a particular level.
If you mean the United States, tell me whether you want to compare basic reading or reading at grade level. Then I can look for the closest trustworthy figures and explain what they do—and do not—show.