Do you have more than one case to generalize from? I have never seen or heard of appetite changing with spay/neutering.
What I have noticed is having to cut back on the food for intact dogs around 7-8 months to maintain good body condition after their growth slows as they near adult size. Failure to do so results in getting fat being blamed on spay/neutering at 6 months.
There’s a plethora of more authoritative cites out there, labdude, I don’t have any at my fingertips. But for sure it’s a given that speutered animals are more likely to gain weight if their food isn’t regulated.
However, since most owners regulate food that really isn’t an issue - adjust food amounts to the individual dog or cat (regardless of reproductive status) and it’s all good. Individual metabolism, activity level, type of diet or food fed - these factors affect weight more than whether a dog is intact or not.
Dogs are definitely more likely to be gluttons than cats, it’s hardwired for reasons in the OP. I’ve owned multiple dogs for decades (and fostered dozens more, as well) and have never free-fed. I do vary meal size though; they seem much more satisfied when allowed to eat a huge protein-crash meal once in a while, accompanied with smaller meals so they don’t get fat. I also have cats; they eat twice a day when the dogs eat.
That has nothing to do with the failure of the owner to maintain good body condition.