Apple

Kind of? But hot take - their format is actually better for flat content. They seem to want people to use their "spatial video" format which seems like it can be just two videos in a QuickTime or MP4 container. It wouldn't surprise me if you could just use ffmpeg to convert whatever into their format pretty dang quickly. It's actually just MV-HEVC.

Most 3D video right now is one video track with two distorted videos either side by side for flat or 180 content, or top and bottom for most 360 content. It gets encoded and played back as standard flat video and then the player does the splitting and dewarping for the headset (or for flat just correcting the aspect ratio). They don't seem to support doing any of that in their built in player.

Instead, with MV-HEVC, they encode one eye as the "main" video track, and do deltas to get the other eye, giving you way better resolution since you aren't splitting the frame in half, and better efficiency since you aren't encoding essentially the same image twice (theoretically you could have a codec that could couple copy a big chunk of the frame like that but I'm not aware of any that actually do). It also means if you play it back in 2d you just get a normal video instead of a weird distorted mess, and you can swap to the other eye if you player supports multi track video. They also do some clever stuff with captions in 3d too.

It doesn't seem like they support any sort of immersive 3d video (i.e. 180 or 360 degree fov) in their player at all, but I haven't looked at it a ton. I mostly just took a glance at their developer stuff. It seems like a very apple thing to do since 180/360 video is difficult to do right.