I couldn't find anything on the great depression requiring pet food to be safe for human consumption. I would love for someone to prove me wrong through.

According to this PetMD article, prior to WW2 pet food was a luxury item. During WW2 pet food production completely stopped due to the war. (No cited sources on this article though, I am not sure how accurate it's claims are?)

According to this Vice article, pet food is held to some standards but technically does not need to be safe for human consumption. (Again, I am not sure how trustworthy of a source Vice is)

This is the best source I have found, a peer reviewed and cited research article published on Jan 30th 2023: Insights-Driven Development of Humanized Foods for Pets. It clashes with the above article in that it talks about pet food production during WW2 and actually cites sources. In regards to pet food being required to be fit for human consumption, it doesn't mention anything about the great depression. It actually mentions that this is a more recent development and that there still isn't a "set standard"

A seemingly simple yet confusing claim that has appeared recently in the pet food space is “human-grade,” whereby additional regulations regarding manufacturing, packaging, and storing of the food ensure compliance with safety standards of human consumption guidelines (Oba et al., 2020). Although this claim has been used for pet foods for a few years, it has no definition in animal feed regulations. The term “edible,” however, is defined by the US Department of Agriculture as foodstuffs that have been processed, inspected, and passed manufacturing regulations that are designed to ensure safety for consumption by humans. Pet foods that truly meet this standard must include only ingredients that are human edible and must be manufactured, packed, and held in accordance with the Code of Federal Regulations (CFR) Title 21 (AAFCO, 2021; FDA, 2022). Currently, AAFCO is in the process of drafting guidance for “human-grade” claims in pet foods in an attempt to provide clarity to the claim (AAFCO, 2021). The proposed definition outlines that pet foods using the “human-grade” claim are first and foremost animal food products that are subjected to inspection under 21 CFR 507, manufacturing must be in accordance with 21 CFR 110, and the overall process is conducted according to standards ensuring human consumption safety (Carter et al., 2014; Oba et al., 2020; FDA, 2022). Furthermore, the pet food bearing a “human-grade” claim must be manufactured in accordance with the applicable human food regulations for ready-to-eat human food (AAFCO, 2021, 2022)