By definition if it remastered, then the original master is no longer the source. Remastering should go back to the multi track recordings and remixes them.
No. This isn't true at all.
Back in the day you could not (or only rarely could) simply transfer a recorded source -- like the original mixdown master tap -- to a consumer medium without any adjustments for the peculiarities of that medium. So a source is 'mastered' for a particular release medium. That is the origin of 'mastering'. From the original recording, you create a 'master' copy that is suitable for production of vinyl, or whatever. This could be a lacquer , or it could another tape, capturing all the tweaks necessary to cut a good lacquer -- a 'cutting master' or a 'production master' tape.
Not to be confused with 'original master tape' (OMT), which *should* refer to the original mixdown tape(s) for a track(s). The OMT is a result of *mixing*, not mastering.
Arguably the advent of CD should have meant that 'mastering' for release would simply be 'flat' digital transfer of source signal (the OMT) to a CD master. No tweaks, just OMT playback from a well-set up reel to reel into a good ADC , then onto a CD metal master disc. But no, mastering engineers still found work 'tweaking' master tape signals so they would 'sound better' on CD. Or in latter days, tweaking it so it would sound good on earbuds in public or in a car (loudness wars). There are of course cases where maybe the master tape itself simply doesn't sound that great. Maybe the engineer and producer were coked out of their heads; maybe crummy studio speakers were used. These do need 'tweaking'.
A remaster *should* mean going back to the source tape -- original mixdown master -- and creating from it a new digital master for release, rather than using a previous digital master. It *could* also now mean going back to a 'flat' digital transfer of the source tape, and creating a tweaked digital copy to use as a new master.
Remixing is an
entirely different thing distinct from remastering. Though of course by definition a remix will generate a new master, hence, a remaster.