The Rambam's Haggadah and the Ba'al HaTanya's Haggadah lack Nirtzah. @DoubleAA quotes a version of the Simanim poem in which the order concludes with Hallel, as well. Also note the discussion here regarding various mesoros about Chasal Siddur Pesach.
Many of the piyutim that comprise Nirtzah are of unknown origin - note Echad Mi Yodeya and Chad Gadya. My question is not about who wrote them. My question is when Nirtzah was widely accepted as a part of the Seder - that is to say, when were these songs tacked on to the end of the Seder?
Related: What is Nirtzah?