He and Kevin Conroy retired the voices of Bats and Joker but came back several times. They retired after Arkham Asylum and came back for Arkham Knight (Origin had different VAs), and they both came back for Killing Joke as well, and their final act together was the Crisis trilogy. The last part is expected to release this year, so we have one last adventure with Kevin’s Batman.
After Kevin’s death I think the retirement may stick for Hamill. He’s said before that he wouldn’t do Joker without Conroy.