They exist, and if you know where to look you will find many of them, almost always not legal. The storage requirements are vast and you will need some pretty specialized hardware to do playback. The biggest reason why stuff is downmixed and flattened is because of our historical distribution media: radio, tape, records, in theory you could distribute the masters but most people would have no use for them.
Some of the youtubers that talk about and analyze music seem to be able to access multi-track recordings of various well-known songs, I was always wondering where they got them from.
Most of them are ripped from Guitar Hero/Rock Band, because the games needed to mute individual instruments when you were not playing the correct notes. (With 10+ games and lots of DLC, there's actually a lot of songs that were included)
There was also a service called jammit marketed to musicians where you could buy multitracks from their partnered artists. There are also artists sharing them in deluxe editions of albums, for example I think Dream Theater did it for Black Clouds and Silver Linings?
There's also the possibility of just knowing someone in the industry that could have access to them, maybe? I know Rick Beato who does song analyses on YouTube is a producer, so he could get some this way.
There was a great channel on YouTube dedicated to sharing these isolated tracks called digital split, unfortunately it got banned from YouTube twice, so I don't think it's coming back...
It wouldn't surprise me if popular channels got access to the raw tracks from the studio (for whatever the normal license terms are to use the full copyrighted song in your video - a share of the advertising).
Popular is key - you need to get popular doing analysis of work nobody has heard of first. Once you are popular on YouTube everyone wants you to advertise for them.
They can make their way into the public's hands in a couple of different ways:
1) The multi-track recordings are deliberately released by an artist who wants to encourage others to remix their music. Sometimes it's just the instrumental. Sometimes it's only the acapella vocals, and other times they release the entire multi-track, or a simplified version of it.
2) There are certain audio tricks that can be applied to separate certain components from a recording like isolating the L or R channels. If you have an instrumental recording, you can use phase cancellation to nullify the instrumental against the full recording end up with just the lead vocals. This technique has it's limitations and isn't always perfect.
3) Box sets for popular albums sometimes deliberately include isolated tracks. The Beach Boys' 'Pet Sounds' box set is a noteworthy in this way.
4) Audio engineers and producers with access to a famous song session or tape reel will copy and leak the entire multi-track recording. This is usually illegal. Nonetheless these are highly sought-after by audio engineers and producers who trade them amongst themselves for their insights into the recording process that they offer.
Rick Beato’s channel is great for this, he’s got multi-track for lots of music from the 60’s/70’s and really breaks them down.
Here’s one episode where he breaks down Ramble On by Led Zeppelin...he says it himself but you can just feel how much he loves the songs as he goes through it:
