We do not find an Amora in all of the Bavli expounding a vav after the time of Rav and Shmuel, who, as members of the transition generation between the tannaitic era and that of the Amoraim, were in a special category (Rav Tanna hu’ u-palig, Ket 8a, Git 38b, BB 42a, Sanh 83b). The system of midrash halakhah was already closing down, though we find that Rava could still expound mi’utim and make use of some other middot.

Yaakov Elman, “Rava as Mara de-Atra in Mahoza”, Hakirah 11 (Spring 2011), 78.