If you search the web about by-products as part of a canine or feline diet, you’ll find reams of negative information which can be summarised as “Do not feed your pet by-products”.
This is misleading.
By-products aren’t bad for our pets! Kidneys, liver, brains, and so forth, are all “by-products” and provide vital nutrients which can’t be sourced from muscle meats. This is something many raw-feeders fail to realise.
The real problem with by-products in a pet food is what those by-products actually are. It’s what’s left over at the very end of the chain after anything which can be sold for human consumption is extracted. What we’re talking about here is waste from the “human consumption” factory thrown into a bin marked “Pet Food Only” (usually inclusive of other general waste from floor staff) which then gets denaturised and sent off to the kibble factory.