Assuming they maintained flocks at all (and fair warning, their expectations were MUCH MUCH lower), you might find
these pages really instructive - (for which I thank
@saysfaa)
Tl;dr? About 80% (mostly junk) grains supplimented by 20% "meat scraps" (no longer allowed inclusion in commercial feed mixes). The grains were largely a blend of corn and wheat, plus sometimes oats and/or barley. Meat scraps were about 55% protein, 2% fiber, 10% fat.
You aren't going to be able to duplicate that if things all go to hell, unless your circumstances are pretty unique. Thankfully, most of us have not suffered complete absence of feeds thru the pandemic thus far, but changing brands, or substituting a starter/grower for an all flock (or vice versa) since they tend to have similar nutritional profiles has been pretty common.
My advice, which you should feel free to disregard, is that if it gets that bad, eat the chicken early - will cost you more resources to keep alive than it produces. and I say that as someone with a stable flock on a potential 30 acres (though only about 1.75a is currently given over toT