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.
I’m finding it surprisingly hard to come up with a simple, concise citation for it. Looking up reviews of individual phones will let you find the exact dimensions of images they capture in various modes, but an exhaustive search would be tedious.
Looking at the product table for Samsung image sensors lists their resolutions, some of which, such as 4000x3000 are obviously 4:3 without picking up the calculator.