Simpsons Question - Grayish black line under the title

Before the words"The Simpsons" comes on screen there is a grayish black line in center of the screen that runs up vertically about 2/5ths of the screen and stays there until the words “The Simpsons” go by. This grayish black line is not there in episodes from earlier seasons. When did this line first appear? It was season 4 at the earliest. What is the reason for the line?

I only noticed it in the last season or two, not as early as S4. I assumed that it was a scratch on the film but then I have no idea if the show is on film.

screen cap?

That is the signal for our alien soon-to-be overlords to begin their invasion.

bump

It’s definitely been around a long time. But not in every episode.

Note that viewing season 1-3 DVDs is of no help: there’s a fancier cloud scene in the opener.