Recently I was reminded of this and I realized I never properly looked into it. What I can say is that at face value, I am wary of these substances because our own microbiome could be affected in 1000 unpredicted ways.
NIH: The Effects on Human Health of Subtherapeutic Use of Antimicrobials in Animal Feeds.
To vastly oversimplify this reading, and to add in a few of my own takes in brackets, due to the huge increase in demand and thus production of animal products which utilizes the factory model, animals live in unsanitary conditions. Thus, it became more efficient to use feeds and water supplemented with antibiotics. As of the publishing of the article, approximately 40% of all antibiotics produced are used in animal feed.
Methods can be taken to reduce the reliance on antibiotics and shift the mechanism to the immune systems of the animals[, however not only could that result in less production and less profits, but also would require the entire industry to change its basic models of functioning]. Wide use of antibiotics is not seen by scientists as a substitute for sanitation and husbandry practices, but rather meant to compliment them. [The industry, however, largely does not follow that practice, especially in countries which have issues with regulatory capture by wealthy industries.]
One significant consequence of using antibiotics is in the development of resistance in bacteria, such as E. coli and salmonella. There appears to be sufficient evidence to conclude that the widespread use of antibiotic additives and supplements has a causal relationship with such resistance, which has significant consequences.
What were less sure about is the direct consequences on consumers of animal products from animals which ingested antibiotics. This question requires additional study and anyone suggesting it’s definitely safe or definitely not safe should be heard with a healthy dose of skepticism.
I’m personally very wary of all these kinds of things. Most of them have not gone through formal regulation and are marked as “generally regarded as safe”, which means that there’s no obvious problem causing a wide spread easy to diagnose issue, but subtle problems or problems that affect only a small part of the population are much harder to prove.