I just watched "The Deal", season two, episode nine. This is the one where Jerry and Elaine decide to have sex, start fighting, and then at the end decide to be a couple.

I don't think I ever saw this referenced again. It was like a one off episode. Obviously they quit being a couple at some point, but did they ever address this in the show? As far as I know, it was never discussed again.

It seems it was supposed to be a season-ender, but aired out of order. The network had been pushing for a Jerry-Elaine romance, so they did the one episode and then let it go.

It seems it was supposed to be a season-ender, but aired out of order. The network had been pushing for a Jerry-Elaine romance, so they did the one episode and then let it go.
Interesting. It just seems kind of jarring for a modern sitcom like Seinfeld to ignore continuity like that.

It's not like the Honeymooners, where there were numerous continuity problems - what happened to the TV? The phone? The dogs? The parrot? - it just wasn't really a concern back in the '50s. But Seinfeld routinely referred back to prior episodes, it just seems really out of place.

Didn't this show find its audience during the third season and explode in popularity?

I'm not condoning the lack of continuity, but if he (Jerry) never wanted his character to be hooked up with Elaine then it would make sense to move forward and never reference it again.

I'm not condoning the lack of continuity, but if he (Jerry) never wanted his character to be hooked up with Elaine then it would make sense to move forward and never reference it again.
Except that they already did it. Even a throwaway line "We just decided to be friends again" would have been better than nothing.

This is somewhat related: Remember in the two part finale when the plane starts to go down? Elaine says "Jerry, I've always..." before it levels out again. There seemed to be an implication she was going to confess her love for him. I guess that sentiment wasn't returned though, Jerry seems kind of numb emotionally.