Per #2 (comment)
Currently, the only way to deliver lossless audio over RTMP is via linear PCM, which has some drawbacks:
- Limited to 16-bit audio
- Max 2 channels
- Max sample rate of 44.1kHz
Adding FLAC would allow some flexibility for stream producers that want to transcode for multiple bandwidths without losing audio quality. Currently the best solution is to max out the AAC bitrate and hope there isn't too much quality loss on transcodes.
Plus, assuming FLAC would signal its own bit-depth, samplerate, and channel count (I know AAC signal its own samplerate, I can't recall if it also signals channel count) - you could stream incredibly high-quality audio.
As far as solutions shipping today that would benefit from FLAC support:
OBS
Say I want to produce a web radio show. Most web radios are based on Icecast/shoutcast and a lot of the tooling around Icecast isn't great.
OBS is actually a pretty decent tool for producing radio-type content with builtin audio filters, support for multiple audio devices, scripting, hotkeys, and so on.
If OBS could output with FLAC over RTMP - a receiving server could take the single RTMP stream, and transcode to different formats (output to Icecast, HLS, and DASH) with significantly less quality loss.
HLS/DASH repackagers
Similar to above - I know the various nginx RTMP add-ons support converting RTMP into HLS and DASH. Being able to accept a FLAC stream would allow for producing HLS, DASH, and even other RTMP outputs with multiple codecs for multiple devices.
Say I want to create a multivariant playlist in HLS with next-generation codecs like xHE-AAC. Having a single endpoint with a lossless codec would allow me to create a multivariant playlist with perfect sample alignment (since it's all from the same source). Currently I need to have my apps produce multiple streams at multiple bitrates and its hard to get everything aligned perfectly, or I have to just stream with AAC and re-encode/repackage server side and lose quality.
Those are two off the top of my head, but it would really expand the possibilities for using RTMP as an intermediary/transport format. A user producing a web radio could stream in FLAC to a server over RTMP, which transcodes to formats like AAC, AAC-HE, xHE-AAC as HLS, and allow the listener to auto-fallback to codecs based on their bandwidth.
Long story short, the audio producer wouldn't need to worry if the final output/destination supports a particular codec - they would just stream in FLAC and have the ingest handle getting the right codecs out to the receivers. FLAC essentially allows nearly any other codecs to be supported via server-side transcoding.