This Odd Lots episode looks at the potential second-order effects of GLP-1 medications (such as Retatrutide) by reducing appetite. One topic they discuss is whether fast food companies are already fretting about these meds in their earnings calls. I thought this episode made a good case that GLP-1 meds could be very important — even beyond improving the well-being of people who directly benefit from them — especially if they become cheaper and more widespread.
I haven't taken any of these drugs, or done much research into them yet, but my impression from this episode (which might be mistaken) is that these drugs can reduce hunger without making food so unrewarding that it becomes extremely difficult to get enough nutrients to thrive. (I would guess, though, that taking these drugs makes it even more important to be conscious about getting all the nutrients one needs.)
Assuming the best version of these drugs can eventually be taken cheaply, sustainably and healthfully, with a low risk of intolerable side effects, a large number of people will want to be on them. If enough of the people who want to be on them eventually can get them, this would presumably reduce the demand for animal farming, if only by reducing the demand for food.
So, GLP-1 meds may end up having some relevance to animal welfare, but I haven't seen animal advocates talking about them yet.
It might also make sense for animal advocates to research vegan nutrition and GLP-1 meds. These meds almost certainly make it more important to eat a nutritionally dense diet. Maybe this could make it harder for some people to be vegan while taking GLP-1 meds. Pre-existing research into vegan diets after gastric bypass surgery could be relevant.