Plex should have an option to automatically find black bars on videos and crop them before streaming it to the client
Otherwise, an option on the client to crop or zoom not to see the bars.
When I have this problem I have to change apps and watch the video on the Plex app for media centers where you can use “z” to change aspect ratio to hide the bars.
This was added a couple years ago. This link describes how to use it on the nVidia Shield, but it should be similar on most apps:
Nvidia Shield black bars - #3 by beckfield
That could be a workaround, but that feature isn’t on the desktop app

The Plex desktop apps (but not the web app) have the most flexible options for this kind of thing.
They only require some bit of tinkering.
Nothing automatic though.
Plex Desktop
Plex HTPC
1 Like
2023 clean-up: duplicate
https://forums.plex.tv/t/automated-border-detection-and-cropping/713154?u=tom80h