Well whether there's a 'movement' or not I probably wouldn't know, to be fair, since I'm not a vet and don't really have my finger on the pulse.... so I should take back what I said and rephrase, there are certainly some vets advocating against it, and there does appear to be some evidence for waiting, or not neutering at all - with male dogs, at least.
A couple of studies for examples, one on german shepherds - https://onlinelibrary.wiley.com/doi/ful ... 02/vms3.34 and one on golden retrievers (which is specifically relevant to me since that's what we have) - https://journals.plos.org/plosone/artic ... ne.0055937
Here's an editorial from a vet which breaks down the GR study above - https://www.gundogmag.com/editorial/con ... mma/176220
These are the sorts of things I was talking about....
You don't think a breeder would have some reasonable anecdotal experience after many years raising litters of dogs, staying in touch with the owners of their progeny, and dealing with vets far more than the average pet owner?
I don't think your anecdote really holds true. They're not vets, sure, and they're not necessarily going to be following 'the science' (although some may well do) but it's far more interrelated than chiros and diet.