Instagram on the big screen is another sign that TV is becoming a giant phone, and creators who shoot vertical video should take note. I think my family will love this, since we share all sorts of video content. Now, that process will be a lot easier. But the horizontal format is something new to think about, especially if you have a creator in your household.
I am the only person in the family who prefers shooting video horizontally, mainly because I use a lot of what I film on YouTube. When you edit and produce video, you can convert portrait videos to landscape relatively cleanly, but it usually means adding a blurred background of the same video behind the main video. You can change landscape videos into portrait, but it requires heavy cropping.
Be prepared to see empty top and bottom sections of videos or heavily cropped videos (or panning and zooming) as creators work to make a “universal” format to work in both modes. The Instagram iPad app has been out for a while, and now that the app is on Apple TV, it will open up yet another way to get content to a wider audience.