For a while I always wondered why Good Neighbors was so hated, because compared to pretty much any season 6-9 episode (haven’t seen much past this) it’s really quite tame. I think the big problem with it is that it isn’t bad in a later SpongeBob way, it just feels like a really bad season 3 episode (for the record I don’t think any actual S3 episodes come close to being bad). Like it *feels* like they’re emulating classic SpongeBob decently but they just can’t quite grasp what made it work so it just feels wrong. Later SpongeBob had consistently way worse episodes than this, but by that point it was just an entirely different show
To answer the question, for me the end of classic SpongeBob has a lot of different answers. As an adult, the most cut and dry answer is the movie because after that the show almost immediately devolved into a shell of itself (even if it wasn’t downright bad yet). I can’t say anything past the movie really feels like classic SpongeBob anymore, even if a couple early S4 episodes come somewhat close to emulating it. As a kid, however, it took me a bit longer to realize the show wasn’t enjoyable anymore. I guess I could tell S4 felt off despite being only around 6-8 years old when it was airing, but I still enjoyed watching it. I think for me though the first episode that really threw me off was “slide whistle stooges/the splinter”. I remember watching it when it premiered, and just feeling so underwhelmed by SWS. It wasn’t funny and went nowhere, and while there were previous episodes that also were like that, this was where I became conscious of it. Following that up with the splinter didn’t help for obvious reasons, another horrid gross unpleasant episode. Season 7’s “Legends of Bikini Bottom” mini series was where I lost all hope. I remember by that point I had proudly seen every single episode, but this mini series really just made me realize I hadn’t enjoyed a new episode in a long time and that I probably wouldn’t enjoy any new ones coming out going forward. I think the episode where Patrick befriends a toxic waste monster was the last one I saw, and after that I sort of just randomly caught season 8/9 episodes when nothing else was on at the moment. Anyways sorry for the essay lol, but yeah around season 6 I’d say was when I stopped liking the show as a kid. Looking at any of the episode titles from that era of the show just give me a deeply negative feeling in the pit of my stomach. Seasons 4/5 are mediocre (*especially* compared to the original run) but are fine children’s filler television all things considered
I feel honestly “Penny Foolish” was when the classic SpongeBob ended. Every episode up to that (even the first 2 season 6 episodes) have a scent of classic SpongeBob. Penny Foolish has the characterisation problems, off putting animation and overall “dumbness” of the dark age. Im probably in the minority of season 1-5 being “classic SpongeBob”
I got to say the episode that got me thinking SpongeBob with changing was the Splinter. Because that's when the show started throwing tons of Bad episodes at you the characters were all becoming unlikable there was constant gross-out humor we had some in the first five seasons but not this bad
