When did A/B ( sometimes C ) stories become a thing ?

My opinion is it was a 70s thing when shows began having larger casts. Early shows like Lucy or Honeymooners focused primarily one or 2 characters. In an ensemble cast everyone needs something to do so shows like MTM, Mash, Brady Bunch would have a main storyline and a back up story. Later shows would have 2, 3 or even 4 plot lines ( I'm looking at you, Seinfeld ! )