Earl Butz and Anita Bryant were also frequent targets of Johnny's monologues. And for a while, they couldn't do a Carnac sketch without mentioning Chuck Barris and/or Howard Cosell. How-wud is still referenced a lot in sports media, but outside of Letterman, and maybe Bill Maher(or Dennis Miller if he had his own show again), I couldn't imagine a current talk-show host mentioning Howard.
Fortunately, Don Rickles is still with us, and occasionally appears with Dave and Jay, or he'd have been on this list a while back. Jokes about Burt Reynolds, Dom DeLuise, Paul Williams, 'Smokey and the Bandit', etc., are pretty far over younger viewers' heads these days.
Bill Cosby has almost reached a point where he's known more for his scolding diatribes against 'today's African-American youth' than for 'The Cosby Show'..and anything he did prior to that('Fat Albert', 'I Spy') is nearly forgotten.