When it becomes clear the writers don't have a long term plan. I've been rewatching Once Upon A Time. Still my all time favourite show but I'm confident that the writers only had a clear plan up to the end of season 3, after that things start to get fuzzy. Storylines get abandoned and unresolved, things happen that seem to be setting up something that doesn't come, storylines happen that almost contradict past events (for example Snow and Charming having a dark secret about what they did to Maleficent, when you watch prior seasons with that in mind, it's really weird nobody brought it up sooner). Yeah, you can just tell they didn't plan past a certain point and they started adding lore that didn't add up

When the potential long-term impact of the events keeps increasing, but the actual long-term impact keeps decreasing.

It's less of an issue in comedies, but main characters becoming Flanderised in drama series is where it becomes obvious they've run out of ideas.

For example, at the beginning of Stranger Things, Hopper had basically given up on life, and over the course of the first two seasons he finds purpose again through helping find Will, and later, raising Eleven as a surrogate daughter.... And then in season 3 he becomes ANGERY MAN WHO FIGHTS PEOPLE - and that's about it.

Loose ends start accumulating and there comes a point where you realize there's no way they could possibly be resolved coherently in the time the series has left. I was feeling this in a big way during seasons 6 and 7 of Game of Thrones.

Depending on the kind of show it is contextual, but here's some.

If it is a tight self contained story that ends...and then more things happen. Stranger Things for example pretty much perfectly ended in season 1. There was a tiny dangling mystery regarding Eleven's fate. Such things do not need to be a sequel hook, they can simply exist to tantalize and never be expanded on. This is like if Inception 2 was made and it answered the questions about Cobb's spinning totem; it would utterly miss the point that the story was over and the ending was intentionally ambiguous.

If the actors or voice actors are simply getting too old for the part. Personally I have a good ear for animation's voice acting. It drives me absolutely crazy when I hear noticeably aged actors reprising role or continuing them as if nothing has changed. Obviously some performers can last longer than others but for example modern Simpsons is unwatchable to me entirely on the basis of the voices. Even if somehow the writing turned around I simply can't get past the voices. Similarly I could barely sit through The Incredibles 2, which supposedly picks up right as the first movie ends but all the voices are aged 14 years and I can hear it.

Back in the 80s and 90s, it was when they added a new child character. In the 2000s, it was when they started doing weird crossovers that made no sense.

Walking back character development/growth.

When the characters are talking about something and say "oh this is like that time when..." and a flashback scene which is just copy and pasted from old footage is used. Then they do this 5 more times in the episode. So annoying and cheap.

When they have a contained shark that the main character decides to jump to keep establishing his cool.

Seriously though, when threat of the week escalates to such a degree that it becomes a potential universe calamity, it's hard to be worried about Murderer McGee stabbing 13 people to death.

Superhero comics are one of those things where I don't think it's possible to jump the shark.

The Justice League itself was kind of an awkward collaboration starting back in the 60s where they brought together a bunch of disparate different comic characters into a shared universe.

That being said, I think a series has jumped the shark when it becomes entirely unrecognizable from its original iteration to the point of absurdity. You would never expect to see a scene where The Fonz jumps over a shark while water skiing if you only saw the first episode of Happy Days

Shoehorning in a new main character

Shoehorning in a new main character to try to increase appeal

Shoehorning in a new main character to try to increase appeal to sell more merchandise

Shoehorning in a new main character to try to increase appeal to sell more merchandise because the higher ups made a decision to drastically change the original main character in an obvious cash grab

I'm looking at you, ya pop tart colored unicorn

The "clip show" is a good sign the writers are running out of ideas. The writers write 10 minutes of dialogue and the rest of the show are scenes from previous episodes.

When the clip shows start appearing.

Identical twins...trapped in the basement...winning the lottery...tonight, on a very special...cousin oliver

When it's based on a book/series and outruns the source material

Meta: when the principal actors become the producers.

When the sexual tension between main characters eventually results in a sexual relationship or they get married.

If it's Cop apologia, when the cop has to go 'rogue' to get 'justice'.

jumped its shark

…. Is this a thing? Origin?

When they introduce time travel to fix the CF of plot holes.

The shows dialogue becomes bloated with convoluted bullshit lines such as "The DOW is over 50,000 right now! It's over 50,000 dollare, you should be GRATEFUL, and we shojld be talking about that!"