Era of Shruti VS Era of Smriti
When there is no gap between listening and remembering, that era is called the Era of Shruti, listening. When there is a gap, incompletion, between listening and remembering, that is era of Smriti, remembrance.
The Upanishadic Era - Era of No incompletion between listening and remembering ends with Rama. After Rama, the words of any Rishi are not considered Shruti, they are only considered Smriti. For example, Krishna’s words are not accepted as Upanishads, as Shruti; they are Smriti.