Starting in 1964, EMI began saving every work tape the Beatles used in the studio, which means that now, years later, they were able to synchronize all the session tapes into a pre-mixdown multi-track hard drive. Every instrument on Sgt Pepper now has its own track, which is why "Love" has a much more discrete mix than they could ever derive from the final 4-track mix-down tape. Most of their albums can be mixed into very good surround mixes, if EMI engineers want to.
Well yes, and no. In many cases when recording to 4 track The Beatles would fill all 4 tracks then bounce these down onto one track on a a new tape. This would the free up 3 more tracks for subsequent overdubbing. In these cases the EMI archives would hold 7 discrete tracks that can be used for surround mixing, or indeed doing a new stereo mix.
The thing is many tracks were not bounced down to a second tape, rather they were captured on one single 4 track tape. In these instances any remixing would be limited by the contents of those 4 tracks.
I've just finished reading a book by Jerry Hammack titled "The Beatles Recording Reference Manual Volume 2 - Help! through Revolver (1965-1966)" It details everything relevant to these recording sessions including instruments, amps, microphones, mixing consoles, tracking machines (tape recorders), engineers, producer etc etc. It includes the detail of exactly what was captured on each track of the 4 track tape.
So taking
Paperback Writer as an example the track contents look like this:
Track 1: Guitar 1, Guitar 2, Drums, Tamborine. These were recorded at the same time. The remaining tracks were filled as superimpositions (overdubs)
Track 2: Bass, Backing Vocal 3
Track 3: Vocal 1, Backing Vocal 1
Track 4: Vocal 2, Backing Vocal 2
So any stems to be used for subsequent remixing would be limited to those 4 tracks. This is also a perfect example of why so many Beatles tracks have the instruments on one side and vocals on the other. There are many exceptions to this of course, but listen to the originals and be surprised how late into their recording career this still happened.