The Simpsons Writers Have Never 'Retconned' Anything From The Series
One of the benefits of writing an animated sitcom like "The Simpsons" is that the characters don't age. And one of the downsides is ... the characters don't age. On one hand, the show hasn't had to struggle through many major status quo changes, like the kids going to college or grandpa passing away. But on the other hand, this also means the show has inevitably started running out of fresh storylines; it's been thirty-three years and Bart and Lisa are still going through the same elementary schoolkid problems, and Maggie's still forbidden from having her own compelling storylines on account of her age. (She occasionally gets fun side stories, but there's a reason why Maggie is no one's favorite character.)
Classic live-action sitcom storylines, like the kids learning how to drive or having their first real relationship, are only possible in occasional one-off episodes taking place in the "Simpsons" future. We may get occasional glimpses of the family at later stages of their lives, but never anything longer than 22 minutes.
The other issue with the characters not aging is that time no longer makes sense here. We've seen the early years of Homer's relationship take place in the seventies in the first few seasons, then moved up to the '90s in season 19. A recent episode has featured Homer as a teenager in the '90s, which serves as a terrifying reminder of how much time has passed for us in real life. Current co-showrunner Matt Selman pre-emptively addressed potential complaints about '90s Homer by tweeting before the episode:
Continuity Alert: Sunday's @TheSimpsons playfully re-interprets the show's timeline to allow Homer to be a teenager in the early '90s — The Simpsons is a 32-year-old series where the characters do not age, so the 'canon' must be elastic/contradictory/silly.
Can't retcon anything if nothing matters
As much as fans were frustrated by "That '90s Episode," which seemingly erased a lot of the established canon of Homer and Marge falling in love in the '70s, it's inevitable that as the show goes on, the writers need to be allowed some flexibility with this sort of thing. Was that episode really a retconning of Marge and Homer's relationship, or was it simply a modern retelling? Matt Selman, at least, considers it to be more of the latter:
"I mean the one word that I really, doesn't factor into how we do the show is 'retconned.' ... We've never once said, 'Hey, we're changing it, this is a retcon!' Like, no, everything happened. It all happened. We're not undoing, we're not writing it over, we're not — this isn't 'Star Trek' or 'Star Wars' where everything has to fit together like a puzzle."
"The Simpsons" has always been self-aware about the way time works in this universe. After a few seasons it was already starting to feel a little weird that Bart and Lisa hadn't moved on to fifth and third grade, so they started throwing in jokes where the characters pointed out how their lives seemed to be stuck on a never-ending loop. In season 5's "Home Loves Flanders," Lisa straight-up tells Bart, "It seems like every week something odd happens to the Simpsons. My advice is to ride it out, make an occasional smart-aleck quip, and by next week, we'll be back to where we started from, ready for another wacky adventure." The episode ends with Homer and Flanders seemingly reconciling their differences for good, only for the final scene to flash forward a week and show that things have inexplicably returned to the status quo.
Nothing matters, so everything does
Sure, if you take all the things that happened throughout "The Simpsons" and tried to imagine them all occurring within a single year of the character's lives, it wouldn't make sense, but that's part of the show's charm. The Simpson family has been through at least 18 Christmases, 33 Halloweens,12 summer vacations, and six presidents, and they haven't aged a day. All of that was real for them. As Matt Selman urged fans to remember:
"It all happened in this one silly world in a silly way that of course doesn't make sense, but it's silly. Like, please, fans, beloved fans, there's no retcon because we're not changing the past. We're just saying it's deliciously contradictory."
At this point, it's best to think of these characters less as real people and more as timeless mythical figures. Homer will always be written like a big talking dog, just like Lisa will always be a voice of reason. Times change, but the characters mostly stay the same. It's this format that's allowed the show to go on for as long as it has, and it's what allowed it to stay fresh for so much longer than the average sitcom. (Regardless of what you think of latter-day "Simpsons," the first 8-9 seasons were an undeniably impressive run.)
Does it make sense that Homer and Marge met and fell in love in the '70s, but then also met and felt in love in the '90s? No, it doesn't, but both of those things happened, and also neither of them did, depending on the episode. The contradictions here might break your brain a little, but trust me: the show's a lot more enjoyable if you just accept it.