First, Elaine isn’t in the pilot of the series originally called The Seinfeld Chronicles because a waitress named Claire (Lee Garlington) was initially supposed to be the “main lady.” After the pilot, however, the character and her restaurant were dropped from the series, in part because the writers wanted a more active member of the main cast (via Decider). In fact, Julia Louis-Dreyfus filmed her first scene as Elaine for the fourth episode, “Male Unbonding” (via YouTube). Mike Richards (Kramer) said he knew the actor was the right cast for the character when she walked onto the set in cowboy boots.
While Louis-Dreyfus has appeared in nearly every episode of the show after that, she took maternity leave before season four so she could have her first child, Henry, with husband Brad Hall (via The AV Club). Elaine therefore does not appear in the season 4 two-part premiere “The Trip,” in which, while in Hollywood, Kramer is accidentally suspected of being the Los Angeles killer, The Smog Strangler.
Otherwise, Elaine has appeared in 177 episodes of “Seinfeld,” and Louis-Dreyfus certainly has no regrets about the role. In 2013, the actor told Katie Couric: “It was just such a joyful thing, you know? It was such a gift and it was so much fun” (via YouTube).