The first Mishna in Shevi'is (I am a bit ahead on the daf) discusses the shiur for tosefes shmittah. This halacha no longer applies because (as the gemara explains) Rabban Gamliel and his bais din nullifed the original takkanah (the right of the latter b"d to do that was built into the original takkanah). Asks the gemara: if the takanah of tosefes was nullified, why is it still recorded in the opening Mishna?
Now, for those of us who are natively bavli-biased, we expect the answer to be something along the lines of saying R' Gamliel is mishna achrona, this is the original takkanah, "v'mishna lo zazah m'mkoma" (as the bavli answers in various places to this type kashe).
Answers the Yerushalmi - just because a flood will never happen do we erase Parshas Noach? Just because there will never be another weekof miluim, do we erase Parshas Miluim?
All I can say is I don't know what to make of this.