I can't say for certain what would have happened if Jerry Seinfeld twittered something people thought was racist -- his liberal politics would probably protect him -- but nobody was going to stop the reruns because of anything Kramer or Newman or the Soup Nazi or Tim Whatley or David Puddy or J. Peterman wrote or said.
Yes it was Michael Richards but he is a key figure in most episodes.
And Jerry said he would no longer perform at colleges because kids were overly sensitive about things he joked about.