Well, it's not unreasonable until you start talking about what spoilers are and when it becomes okay to talk about. I didn't see the thread title in question, but apparently it involved publicly known news about someone being in the movie and a mention that there's a twist. For most people, that's not a spoiler. "Edelgard deserved better" is also not a spoiler. Similarly, people here genuinely being like "I'm going to watch X show/movie months/years after release so no one can talk about it until then" like... Spoilers: Darth Vader is Luke's father, sorry, y'all.
No one is saying people should be putting THE BUTLER DID IT in thread titles immediately after release. Just that if you're the outlier in what you consider spoilers, perhaps that's on you to navigate your life rather than on everyone else. FWIW, I can't recall seeing any egregious spoilers in thread titles on era.
It's not about breaking down what is and isn't a spoiler, and trying to navigate that minutiae though. You don't have to handwring trying to figure out what a spoiler is for others.
The point is that it's not hard to come up with a thread title that indicates conversation in the thread is going to contain spoilers.
I haven't created any threads on Era yet, but in the past, before I create threads, I ask myself, "does this thread title need to be that specific?"
"Let's talk about Vader being Luke's father in The Empire Strikes Back"
compared to
"SPOILERS: Discussion about the big moment in The Empire Strikes Back."
or
"SPOILERS: Foreshadowing in The Usual Suspects (Kizer Soze talk)"
Personally, I don't care if it's a new or old piece of entertainment. I don't live in a bubble, and there are a lot of people that haven't been exposed to every piece of entertainment out there. It's not some big mental contortion I have to do to keep spoilers out of a thread title. All a thread title needs is the subject and purpose. Any extraneous details can be saved for the actual OP in the thread itself.