Obviously peoples idea of customs differ, and peoples experiences are clearly going to differ in a number of ways and reasons, and some may not be tuned into to "how it's done" even in their own neck of the woods.
I can tell you with absolute certainty that in my neck of the woods, it is ABSOLUTELY customary to feed the help. Photographer, video, DJ, etc.
I can also tell you that the client would not even need to be involved, other than the 10 minute break, as around here the caterer, bar tenders, photographers, etc, all tend to watch out for eac hother. The caterer that gives me a meal, the bartender that provides me with a cold seltzer, are going to get my recommendation. It's good business to take care of each other. Referrals happen.. The Caterer would positively insist on feeding them.
And having been in the event business and venue business for a good deal of my life, I have never encountered a caterer that was not prepared to feed a good half dozen to a dozen more people than what was ordered by the client. That's just standard preparation. No, in fact to not be prepared for that uncertainty (or certainty!) for an extra handful of guests, or feeding some additional people, would reflect very poorly on the caterer.
"Sorry I ran out of memory cards, i can't take any more photos" .. not something you'd here from the photographer. "We can't feed anyone else" likewise, you don't hear that from caterers. At the end of the night, there is always extra.
This is how I KNOW that the Newlyweds in this case were just not familiar with custom. that "$XXX.XX per plate" line means they did not discuss it with the Caterer. I would bet on it. It was a knee jerk reaction from someone that felt overly entitled to the "helps" time.
I come from this from the P.O.V. of a professional event manager, a Venue operator, a wedding client that OF COURSE fed the photographers and the band, and the trivia jockey, (and well anyone else that our excellent caterer decided to feed!) and yes, on the rarest of occasions, I've even been a wedding photographer (ok 3, but still)
The answer to the OP's question is: YES.
We can debate the realities of contracts, and discussion of the meal period ahead of time,. In fact I would highly recommend that the client be aware that the photographer needs to get that brief break and sustenance. I suppose it is within the realm of reality that some newlyweds may not understand these basic bodily needs and or niceties,..
But that is not really the question. The question is if it is customary,.
...and as our own pole here indicates clearly, this is simply how it is done! (again, at least in my part of the world, and yes that extends to Ontario where the OP posted which is also my part of the world. In fact having spent a good part of my life in Ontario Canada and i would find it even more likely there.)
Are their exceptions to this rule? Of course, apparently some people will even cut you in line. But the poor behavior of the individuals living on the fringes of civilized society do not get to make the rule. They can only be the exception.
The line cutters, the road ragers, those that don't tip, and yes, the internet trolls, none of these people make the rules, or the customs.