The Germans in desperation to defeat the Soviets unleashes chemical and biological weapons on a large scale on the Eastern front. The Wester Allies on seeing the cold ruthlessness of the dieing German regime hardens their hearts and decides to turn Germany into a wasteland that will 'Never again threaten civilized countries' The Soviets are greatly weakened by the disease and death inflicted by the Germans. The U.S. and Britian are reinforced in their decision to implement the Morganthau plan when the come across the death camps in Germany
The Germans didn´t do that in OTL so you´d need a plausible POD why they would do it here. And why they thought it would help them?
If so they could use nerve gases. An (only short) Internet search says that they didn´t develop any offensive biological weapons against humans? Foot and mouth disease might be a possibility though.
Gas warfare though only makes sense against large concentrations of people (soldiers) near the frontline (using artillery guns). Since Germany later in the war didn´t have the capacity for large scale (never mind long range) air attacks. So any attacks will "only" hit land already ravaged by the war. No way to touch the Ural tank factories for example.
There´s also the fact that German artillery is hopelessly outnumbered. If they start firing they´ll start seeing counter battery fire pretty quickly. Just imagine what happens if some of the stored gas shells get hit? You think German soldiers back then had the antidotes or had full anti chemical suits?
Not to forget that the agents in the shells degrade over time. For some of them it´s just weeks or months since binary chemical weapons aren´t yet developed. So quite a few of the shells will be duds.
Or how to get the shells to the frontline. A random air (or partisan) attack on one of the trains might prove "problematic" so to speak. Transporting lots of nerve gas shells to the frontline also means that other supplies (ammunition, fuel, spare parts) won´t be transported since Germany only has limited supply capacity.
Or how the Red Army would respond. I assume they have their own biological and chemical programs?
So a German desperate large scale chemical attack on the Eastern front strikes me as not really plausible? A surprise first attack with chemical weapons might kill a lot of Red Army soldiers, true. But to do that you´d need to reduce the transport of all other supplies needed by the army. Doesn´t seem really smart? During the time you transport the chemical shells, the German armies in the East get even less supplies than in OTL? You might transport gas shells to locations already overrun by the Red Army because the German units there had less ammunition.
And in return the German army from now on gets hit with Soviet biological and chemical weapons.
Not to mention that it probably won´t really weaken the Soviet Union since Germany can´t hit the factories in the Ural region for example.
Oh, and when the US and Britain come across the concentration camps in Germany (the real death camps were in Poland) they´ll find quite a lot of Germans there too. From Communists, Social Democrats, Catholic Center politicians to trade unionists, clerics, writers and artists. Just to mention it.
And if the US and Britain implement the Morgenthau plan they´ll be responsible for the death of up to 25 million people. From a newborn babe to the old retired people. Good luck selling that as the "good war".
By the way as far as I know, the Allies didn´t allow international aid organizations to enter Germany till sometime in 1946? Even deliveries of baby food donated by the Catholic church weren´t allowed into the occupation zones.
So late 1945 / early 1946 seems to come pretty close to your scenario? High death rate among babies, sick people, old people?
Just let it go on for - I don´t know? - 5 to 10 years then the surplus 25 million Germans might be dead?