This is a non-exhaustive list, please feel free to add links and comments
Littlstar : Audio Specifications not listed at this time
B-Format (with flavor)
Jaunt (FuMa) : https://www.jauntvr.com/creators/submissions/specs/
YouTube (AmbiX ACN SN3D) : https://support.google.com/youtube/answer/6395969?hl=en-GB&ref_topic=2888648
Samsung VR (AmbiX ACN SN3D) : https://samsungvr.com/portal/content/content_specs
Facebook 360 (.TBE, AmbiX SN3D + Headlocked stereo) : https://www.facebook.com/facebookmedia/get-started/360
Semi-related: the AES has released preliminary guidelines for OTT and streaming applications.
On the topic of loudness:
1. For delivery of OTT and OVD content, where the distribution system supports loudness control metadata, loudness metadata should be encoded in the content bitstream reflecting the value of the measured loudness of the content.1, 2, 3 Where there is no prior arrangement by the parties regarding loudness, the appropriate broadcast regional content delivery and exchange loudness value recommendation, typically –24 LKFS, –23 LUFS, should be followed.
2. For delivery of OTT and OVD content without loudness metadata or where metadata support by the distribution system is not present or is uncertain:
For delivery of content to devices capable of full dynamic range, the content should be prepared using the regional broadcast loudness standard for full range devices.
For delivery of content to devices with limited dynamic range and for use cases with expected limited dynamic range, it may be necessary to pre-process content prior to encoding to accommodate devices with limited dynamic range and loudness management capabilities. In this case, average content loudness should never exceed –16 LKFS, –16 LUFS.
If the employed distribution format supports the carriage of loudness metadata, loudness metadata should be encoded in the content bitstream reflecting the value of the measured loudness of the content.