See Provided Link
21
25
season 21 is 2009-2010
See Provided Link
21 eps .
Yes. This happened in Season 23.
Season Nine's Trash of the Titans
Season 29 began on October 01st 2017.
The Simpsons were featured as animated shorts on The Tracey Ullman Show prior to the start on the series.
The episode order is typically 20 to 22 episodes a TV season.
Because Pokemon episodes take less time to produce, plus from what I can find online, the episodes are made & air year round with a few breaks here & there. The Simpsons however only airs new episodes from September to May & only 20 to 23 episodes air each television season.
There are zero unaired episodes of The Simpsons. I suppose technically the newest season 29, which debuted on October 01st 2017, is filled with 'unaired' episodes but naturally there is a distinct difference in episodes that were supposed to air & didn't and completed episodes that haven't been scheduled to air.