The slag was entrained during ingot pouring. (I'm not aware of any continuous casters back in the 1970-'s)
Or it could have been insufficient cropping of the ingot top.
To your point, from a routine product check chemical analysis, yes, the material probably met specification- for chemical analysis.
However, for Entrained slag this was likely rejectable only upon finding it and calling it an injurious defect, there is no requirement in ASTM A6 , 131, or others that called this out specifically at the time.
Now, ASTM A6 General Requirements for rolled Structuiral steel bars, plates shapes and sheet piling, Supplemental requirement S8, Ultrasonice examination can be specified to assure that any material with such slag entrainment is rejected.
Semantically, one could argue that the steel containing enmtrained slag is not pure steel, it is steel contaminated with contained slag nonmetallics. And so lacking in 'Purity.' But since this is detected by physical means (UT inspection) rather than chemical analysis, we tend to not look at it as a chemical issue as much as an "internal cleanliness / homogeneity" issue.
But your point is exactly right using the lens of ASTM A6, section 10 Quality, item 10.1: the plates you described do not meet the quality standard of being "free of injurious defects and shall have a workmanlike finish..."
Specifications for steel products need to cover more than just chemical composition: mechanical properties, compliance to required processing paths (ie q&T rather than just heavy Cold work, compliance to dimensional and reduction ratios specified, and supplemental requirements for metallurgical structure- Grainsize, and metallurgical structure, frredom from nonmetallic inclusions beyond acceptable levels, Impact test minima Weldability, Bendability, and even how the product is marked and packaged and other requirements not known to me (flatness, straightness, square of cut, type of cut, whether or not it was vacuum degassed, etc. etc.) have a place in determining whether a shipment is a "quality delivery" or not.
People say between two opposed opinions the truth lies in the middle. Not at all! Between them lies the problem, what is unseeable,eternally active life, contemplated in repose. Goethe