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.
That depends on the phone. I remember some old Samsung flagships using a native 16:9 sensors. That was over at least half a decade ago though. Another device with such a setup I remember would be the Nokia 8 Sirocco.
And, the now extinct Windows phones had atypical arrangement where neither aspect ratios would result in the full usage of the sensor, though 4:3 got closer. The Sony Xperia 1 V has multi-aspect ratios too, afaik, where you’d gain some horizontal FOV at the expense of the same vertically when switching from 4.3:3 to 16:9 or longer.
For the vast majority of the phones and their users out there though, I agree with you. I’ve seen way too many people go with 18-21:9 pictures, probably thinking they’re capturing more of the scene as they see that aspect ratio fill up their screen.