I am not 100% certain but I believe that commercial chicken feed has whey as an ingrediant. In some countries geese and ducks used to be fattened on soured milk. I would wonder if the OP had noticed any problems with his chickens. Some bird feeding books still suggest feeding scraps of cheese to birds in the winter to provide fat. You have brought up an interesting suject which I will have to check on more fully.

most anything with fat in it will get stale even in a freezer. But almost all birds have a very acidic digestive system, birds can get away with eating road kill and insects and garbage because of the high acid breaks down and digests food within hours of eating. most any bacteria or mold stays dorment in the cold freezer and the suet will probably be gone long before it could spoil. the worst time for any bird seed or foods is hot moist weather where thing get mold or fungus a lot easier.

i have googled use of bacon grease and also bing search and asked about it with a bunch of different audubon societys in different states. the majority of them recomend useing bacon grease as long as the birds have sufficient water supplys. I have used bacon grease for many years and have lots of birds but i do keep fresh water or heated water trays in winter. I usually mix equal parts of suet or lard or bacon grease or tallow or shortning with peanut butter and add some chopped nuts and then thicken it with cornmeal and rolled oats or grits or flour to make a paste. we don't eat a lot of bacon or fatty meats anymore but do save the fats and freeze them. and mix whatever i have on hand into the mix and the bacon grease is usually pretty well diluted down. Most say it attracts the woodpeckers and chickadees and nuthatches but i get almost every kind of bird i get eating mine. including the jays and juncos and finches ect.

i just warm up the grease or lard enough along with the peanut butter in the micro wave to be able to mix in the other ingredients in a glass bowl. by the time i get it mixed up it's already cooled off some and i spoon it in the lid or tray and mash it flat.I leave it on the counter for a little while so it's not runny and then put it outside. I usually make up extra ahead of time and usally freeze the ectra trays till i am ready to use them