Great resource for high quality landscape posters Apple TV Shows & Movies Artwork Finder by Ben Dodson
This is a future-proof feature IMO. There is no such a thing as a standard movie poster, there have always been various sizes and styles for posters in the industry. Furthermore, streaming services now use both portrait and landscape interchangeably, and may add other ratios in the future.
For reference, you can take a look at Italian Posters - Posteritati Movie Poster Gallery for historical posters that donāt fit in the vertical/landscape categories.
Therefore, I think it would be a mistake to solve this as a vertical/landscape binary. Then, if you take a look at the Apple ecosystem, the Music app imports album artwork respecting the ratio of the artwork cover. That way, for example, home rips of music released only on the cassette format doesnāt get squashed nor cropped (apologies that I only have Mojave at hand):
These are populated nicely along with the more typical square cover on the Music app. Likewise, rows of movie posters in various ratios could look just as nice on Infuse.
I do love Infuse, but I still wish that there was a browsing layout which used landscape artwork. I use list view, because then the thumbnails Iāve reformatted from YouTubeās 16:9 to 2:3 by adding black bars at the top and bottom are a bit larger than in the grid view. But list view has loads of space which could accommodate 16:9 artwork too!
Donāt forget that you need to click the like button on the first post in this thread to show your support for this suggestion! 
The Apple TV app displays a selection of movies and series in landscape mode that advances dynamically, and you can also slide. When you stay on the movie, the trailer starts playing in the background automatically, and if you want to watch it in full size, you slide up. Itās great! Iād like to have the same functionality.
Please give us the ability to choose artwork orientation. Poster or Thumb Nail (Landscape.