The title of this post is "How to upload Stereo 3D (Flat SBS) Videos?", but that question was never actually answered. @DraiKo may have been able to fix the video settings to display it properly, but that does not help video creators on this forum who want to upload Flat 3D SBS content. I know DeoVR admin/moderators have a different set of video tools than users. For users, for Video Type, we basically have to choose between "Flat 2D" or "SBS". But there SHOULD be another option: "Flat 3D: SBS"
I have been emailing back and forth with DeoVR support for over a month and there still is not a clear solution. I was originally told to choose "Flat" and "View Angle: 120", but that did not playback the videos in 3D. Admin had to change my videos to "SBS" and apparently have a "Flat" checkbox on their webpage to make sure the video plays Flat and not Spherical (like a VR180 or 360) This process needs to be much simpler and streamlined and shouldn't require DeoVR mods to have to change settings. All the settings should be able to be entered during the video upload process.
DeoVR also doesn't specify if they support Half-SBS or Full-SBS 3D formats. I uploaded a few Half-SBS videos and it took DeoVR admins a few days to get it to playback in the correct aspect ratio instead of squished. (but they did not tell me exactly how they fixed the videos which is not helpful) With some trial and error on my own, I believe that uploading a full-SBS 3D file with a View Angle of 180 is the best chance of getting it to play properly.
No where on DeoVR's official documentations/guides do they even mention Flat 3D videos. Hopefully they make this platform more Flat-3D friendly before the Apple Vision Pro headset releases or else they will have users trying to upload their Spatial Videos (which are essentially 1080p Flat SBS 3D files) to DeoVR and having all sorts of playback issues and confusion. It doesn't help that the upload page uses confusing abbreviations that most users will not understand such as "sbs2l" or "ab2l". Why not just spell it out in full english? "Flat 3D Side by Side: Left/Right" "Spherical 360: Top/Bottom" etc.
Lastly, the DeoVR app has tons of Filters and categories to choose from "VR180, 360, 8K, Travel, Nature", but they don't have a dedicated "Flat 3D" category. There is no way to filter out the site for just Flat 3D content. Typing in "Flat" into the search bar will bring you a mix of 2D, 3D, and some VR180 videos for some reason.