I noticed a few episodes of Welcome Back Kotter where the actor was credited but wasnt seen in the episode

That scene must have been cut.

That scene must have been cut.


And that can't even be confirmed without a original airing

That scene must have been cut.

No, in the case of Welcome Back Kotter, season 4 was notoriously Kotter-free.

Gabe Kaplan had issues with the producers and essentially went on strike. He was in something like 8 or 9 episodes out of 23 that whole final season.

But, he had signed a contract, and was credited as the star for every episode, even the ones he wasn't in. And that's how it is with contracts for shows. Even if an actor isn't in the episode, if they're a contracted star they will be credited. Another example is Cirroc Lofton on Star Trek: Deep Space Nine, whose name appeared in the opening credits of all 176 episodes, even though he appeared in 75 of them in all.