let the river be the dividing line and put an end to what started out as a civil war...and it still is...
Militarily, the steppes North of the Black Sea - which include most of Zaporizhzhia oblast, is incredibly vulnerable to a Ukrainian drive south because there is no real natural boundary. That's what Russia would end up losing next, and then Kherson oblast is cutoff.
Ukraine will never accept the loss of either Kherson or Zaporizhzhia, though they may be willing to discuss some arrangent within the Donbass. So if Russia isn't willing to accept that, the war can and will go on.
If this truly is a Ukrainian proposal, it is a very smart move politically. It will be viewed as a reasonable offer by those in the West, and if Russia rejects it, that will increase the likelihood that the West will continue to support Ukraine militarily.