Correct. TV Mode is in Plex Media Player. It has a layout similar to the Plex Smart TV / Android TV / etc apps.
I just looked at Plex for Windows. I do not see an option to do what you want in Settings or during playback.
Edit: FYI, Plex for Android & Android TV has the desired capability if you’ve one of those devices (phone, tablet, Shield, FireTV, etc). See this post. Choose Original (if I remember correctly) instead of Letterbox. Not sure if Smart TV apps (LG, Samsung, etc) have this capability. I’ve never checked.
