Do Netflix original films / series stay on Netflix forever or do they disappear at some point?

I don't actually do anime, but Beastars is awesome! I'm the biggest fan ever! And I'm afraid that it will eventually disappear because season 2 is coming in summer

No come disappear some have also been removed from the catalog like knight of sedonia

So if netflix is all right about a film / series it is illogical that they "take it down" again because they then lose possible money. If you take it down for whatever reason, every series can also be streamed on any other portals.

They stay forever.

Is not a Netflix original series.

Okay thanks

Really not, I could have sworn she would be

Netlix also calls something "N-Series" when they bought the exclusive rights. So the only ones who stream it in the country, even if they didn't produce it themselves. Knights of Sidonia was such a case.
For example Babylon Berlin is an "N-Series" https://webcache.googleusercontent.com/search?q=cache:QbBTFBhnj6cJ:https://www.netflix.com/title/80136321+&cd=1&hl=de&ct=clnk&gl=de in the USA
The series is not available from Sky Germany or on German Netflix.

You have to differentiate between real Netflix Originals and those that Netflix simply calls that.
Real Netflix Originals have been produced / commissioned by Netflix. They stay forever. Because Netflix is the rights holder.
Other series that Netflix calls "N series", but are only exclusive purchases, they will disappear because Netflix normally bought a license there that does not last forever.

So sorry to tell you, Beastars isn't a real Netflix original. The series is from Fuji TV.

Wikipedia has good lists of which are correct and which are not.
Correct Netflix Originals (series): https://en.wikipedia.org/...rogramming
Correct Netflix Originals (movies): https://en.wikipedia.org/...inal_films
Real Netflix Originals that have already ended: https://en.wikipedia.org/...rogramming

And here are all of the fake Netflix Originals: https://en.wikipedia.org/...rogramming

😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭😭

At least until summer the series will stay because of the second season, but does that mean I can never see them again or will Dan come back sometime? 😭😭😭😭😭

As far as I don't know, Netflix has announced that the license for this series will soon expire. Series / films that disappear are for example here https://de.flixable.com/leaving-netflix/
Expiring licenses through to summer and beyond, Netflix never says. So you can't tell now when it will disappear.

If possible, Netflix can also renew a license. Sometimes Netflix does this silently and we don't even notice. Sometimes they take down a series, but it comes back later. Some don't come back at all.
Whether Netflix will renew a license or buy it again at some point depends on whether the series was successful.
So no one can really answer that here.