I had the top 75-100 songs for each year from 1960 thru 1995, and when we were forced to switch to YouTube, I started all over again from 1960 giving each song the new YouTube URL.
6 months or so later when I was playing the '70's, I went back to play some 1965 songs, and they were no longer playable.
After about 3 days of raging anger, I calmed down enough to realize that most were deleted due to copyright violation, poster request, etc., but that still didn't/hasn't explained the rest.
I gave up making lists to keep for any length of time.
And that's why we never tried to index the songs again.