Why YSK: I’ve been seeing an increasing number of phone photos shared online in 9:16, 9:21 or similarly tall aspect ratios, often with parts of the subject cut off. I’ve asked a few people why they cropped their images that way, and none of them knew they were cropped.
One issue with shooting uncropped is that the composition will most likely take advantage of the aspect ratio you are using and not necessarily crop well. Composing the shot in the aspect ratio that you intend to display it is actually a better technique unless you are very skilled and able to imagine how you’re going to crop it in the future.