I don't like arguments of the form "tons of people are tweeting that Tom Brady's arm is sh*t, so what's wrong with criticizing Mary Cain's weight?".
Tom Brady probably makes more money than all pro US runners combined. 100X more people know who Tom Brady is than know of Cain, Centrowitz, Huddle, or any US runner. Tom Brady could not possibly read everything that people tweet or write about him. He can afford a therapist for his therapist. I think he's a public persona in a different way than just about any runner (perhaps with the exception of Bolt in an Olympic year).
I suggest the following for talking about elite runners: No discussion of weight for any individual who's not getting paid (which includes all high school and college runners). Limited discussion of weight for people who are in a healthy weight range but not at optimal racing weight. No criticism from photos, like "XYZ's looking heavy, no wonder they're slow this season".
I think it's a bad look for Let's Run and the running community as a whole when there's a bunch of men on here discussing the weight of young women, especially high school/college/amateur athletes. If I were a pro, I'd absolutely avoid reading any thread here about me. If posters here are noticing the weight of a pro, then certainly the athlete is already aware that their weight is unusual for the performance level they want to be at, and they're likely already struggling with weight.
I'm perfectly fine with discussing weight with forum posters asking for running advice or who share their weight at all. I'm also perfectly fine with body discussion that's not tied to an individual like "ideal 1500m race weight for 5'5" woman" or "how much faster will be marathon be if I drop 5 pounds".