One thing that may have made this difficult is that, in fact, there was never a time when cable didn't have ads. It was invented to deliver standard (advertising laden) TV to places geographically out-of-reach of broadcast, and basic cable always was a mix of broadcast stations and additional ad-laden stations.
Premium cable channels didn't tend to have advertising except for their own (or shared corporate parent) programming, but those were charged additionally on top of basic cable.
As someone who had cable since the mid-1980s, its been really weird to see this recent invention of a lost past where cable existed but was ad-free.