Are there any known ways to undo the compression? Assuming no clipping, the process should be reversible, right?
If you just amplify the whole track until its max amplitude reaches the medium's maximum, yes you could undo that.
But the loudness war aims to make the whole track even louder than that, by quietening those max peaks so they don't clip, then that gives you room to amplify the rest of the track even further. The dynamic range of the recording is permanently reduced.
same here, but there is no real market for somebody to bother yet
That's a massive stretch.
They are able separate them sort of, but they are nowhere close the original quality of the individual tracks.
and true in practice too. what you get out of an ai demuxer isn't an exact match for what went into the mix. it's a plausible approximation, much like ai upscalers, upsamplers, colorizers, and such.
In the open metadata world there is ReplayGain which analyzes music peaks and tries to create a negative gain to equalize the dynamic range to a standard volume at both the individual track and full album level.
Apple Music, Spotify, and others have proprietary but similar systems.
(As someone who deeply loves to shuffle an entire library, having a music player that supports ReplayGain has long been a personal requirement.)
On Windows I've always liked Foobar 2000 for its strong ReplayGain support, both automating the analyzing pass and respecting the metadata in the tags once saved. On Unix I was using Banshee for playback and automating analyzing with a pair of CLI tools I've forgotten the names of, one was MP3 specific and the other Ogg/FLAC-specific, as I recall.
If you remember making a playlist where one song is suddenly much louder than the last, and you’re riding the volume knob on every other song, you’ll see why this is nice!
These tools are most useful when used earlier in the process. Like when you just tracked an amazing vocal take, but the gain was too hot on one or two notes. The tools can mitigate some of the distortion artifacts to make it more usable. Applying these tools to complex material like a full mix will have some improvements, but at that stage there's less guarantee for convincing restoration of the record.
What I think non-professionals don't understand is that a record that is characterized by heavy compression is not something that happened at the very end with the mastering stage. It is an aesthetic choice that was made dozens of times along the way while recording, arranging, and mixing. Heavy compression is not necessarily a bad thing. Lots of amazing-sounding records harness it well. It's an art AND a craft. It takes audio engineers and producers years to do it well and with taste.
For analog there are similar limitations, but it's limited by other factors like noise.