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.
If that’s your preference, then that’s what you should do.
I’m seeing an increasing diversity in screen aspect ratios lately though, so it might be worth considering whether that’s the aspect ratio you want to use for a photo you’ll care about longer than your current display devices. If you shoot it uncropped, you can always create cropped copies later.
WRT diversity, my current laptop is 16:9. My next one will probably be 3:2. My main external display is 16:10, and the secondary is 9:16. My phone is 19.5:9. Media rarely fills the screen for me.
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.