Here ya go, Rockhoven and BSG
What better place to compare notes (pun intended) on what is and is not written down, than on a notation software site.
There are solo concertos that come down to us from Baroque time that are little more than a melody line over figured bass. Players knew what to do with it much like contemporary musicians know what to do with a lead sheet.
Historically it's not accurate to say that rock or pop music is the classical music of today. There has always been music for the general population that was much more popular than "classical" music. Classical music has always been meant for a particular (small) part of the population. In other words, classical music has never been popular, as in liked by a large segment of any population.
It is my understanding that sitar has traditionally been taught by rote.