He is talking about the practice of ornamentation, especially as it changed from the Baroque to the Classical periods. In the Baroque, nearly everything (except Bach) could be ornamented. In the Classical period, almost nothing could. Mind you, there were occasions when Mozart provided ornaments for some of his pieces, so that's a rule sometimes broken. But look at those Handel ornaments! I once heard the singer Nigel Rogers give a concert of 17th century songs and he ornamented them to that degree of complexity. At that time, the mid-70s, he was probably the only singer in the world who had the technical ability to do so. He ended up having to do seven encores! So, it is to be suspected that even the most outrageously ornamented performance today probably falls far short of what might have been done in earlier times.
As Rosen points out, ornaments, except in opera and a few other places, like concertos, became less common in the Classical period. Composers achieved their expressive goals with different means: rhythmic contrast, harmonic variety and melodic subtlety. If you really look closely at their music, you can't help but regret a bit that we have lost some of that subtlety. Yes, of course we have a far wider range in terms of rhythm and harmonic dissonance. But the sheer grace we can see in, for example, Mozart, is almost unattainable in the musical vocabulary of today.
Here is Daniel Barenboim playing that Mozart Sonata, K. 457. I believe the ornamented passage shown above is just after the 10 minute mark: