In Season 18, Episode 7: Ice Cream of Margie (with the blue hair) - Is there a reason that they removed the scene where Homer puts on a slow motion record of the ice cream trucks song while kissing? Is it because of time constraints? I'm not sure why that scene isn't in any reruns of that episode. Can anyone clarify why?
There's a detailed guide related to every syndication cut made for episodes of The Simpsons. The cut mentioned in the original question appears to be referenced. Simply search for episode production code HABF22, find the fourth entry and look for the section marked Act II to find the referenced scene.
The entry specifically:
ACT II <0:27> (0:16) 5:38 / 5:54
Homer and Marge start kissing in bed. % Closer on Homer: % % Homer: [laughing] Oh ho ho, let me just put on some romantic music. % % He reaches down to a portable turntable, and puts on a 45 of "Pop % Goes the Weasel" (truly the forbidden song!). They kiss more. <0:27> % % Homer: Hey, we got the whole night, baby. Let's slow things down. % % He turns it down to 16 rpm. % % Marge: [gasp] Ooh... % % More kissing. Later... Texan: Marge, you're one fantastic artist.
Now for the more detailed aspect of the answer! As time has gone on, the amount of commercials aired during a TV show has gone up.
Back in the 1960s, an hour long program had a running time of 51 minutes, with the remaining 9 minutes usually being ads. Today most hour long shows only run 42 minutes, with the remaining 18 minutes being ads. The same is generally true for half-hour shows, which typically ran for 22 minutes of content, but as of late are sometimes shortened to just 19 minutes of actual content with the rest of the time slot being ads.
As such, older episodes of programs have to be pared down in some way to meet the advertising demands of today. This means that some content needs to be cut, so editors will look for bits of the program here and there they can remove without harming the overall continuity of the story. This is called a "syndication cut", while the original is called Sometimes networks will even speed up episodes slightly to keep all the content, but allowing for the inclusion of more ads, as is the case with most Seinfeld reruns on TBS.
This is why sometimes you'll see disclaimers before movies shown on TV and even some older shows that say something to the effect of, "This program has been edited for content and to run within the time aloted."
I can also confirm that episodes aired during special event marathons of The Simpsons on FXX are the original broadcast cuts, not the syndication cuts. My local Fox station would air episodes every weekday from 5 to 6 for years, allowing me to become very familiar with syndication cuts and commercial timings, and while watching some of the marathon in the last week or so I've definitely noticed jokes I hadn't seen in a while or have no memory of.