I’m a little late for the spoilers discussion, but in the context of this thread I think it’s perennially relevant. I also think it’s pretty clear-cut, and that some of the common arguments don’t really hold up.
There’s no statute of limitations on true spoiling, which I would define as revealing so much that you’re possibly lessening someone’s experience.
At the time of its release, we couldn’t escape discussion of and reference to The Sixth Sense. When it’s that ingrained in the wider culture, it’s harder to argue that you have to refrain from talking about it - at least, after the video release. It’d be nice if people didn’t blatantly spoil but it was hard to avoid being spoiled in that case, because people are rude.
It’s now nearly 20 years later (yikes) though. It’s no longer so deeply ingrained in the culture that being spoiled is inevitable. If I were to discuss any twists or catchphrases from that, I would make sure everyone had seen it first. There are millions who haven’t seen it yet, many of whom weren’t even born when it came out and are only now at an age where they can enjoy it.
It’s easy to forget this. I think of my friend’s 10-year-old daughter, who is getting into movies (including old ones) - I want her to have the whole experience I got to have watching a movie like that, or any other. It’s surprises like in that movie that make people fall in love with movies, and I want to encourage that.
It’s just common decency not to outright reveal information that could potentially lessen somebody’s experience of watching something for the first time.
This applies even to things that have a huge cultural impact like The Sixth Sense or The Usual Suspects or The Empire Strikes Back or Casablanca even, it applies to things where the plot doesn’t have twists and turns and may seem obvious like romantic comedies (is this obvious to a 10-year old?), it applies to things that don’t even really have a plot (the WW1 ending of Blackadder comes to mind), and it absolutely has no statute of limitations.
Witness for the Prosecution is twice as old as I am but you’d have to be a total dick to reveal the ending to someone who hasn’t seen it, particularly a young person. I mean that’s enjoyable even if you do know the ending, but being genuinely surprised is a relatively rare and wonderful thing that makes movies enjoyable.
That’s not to say that you can’t discuss spoilers - obviously you can and should (I enjoyed the TLJ discussion). Just state at the beginning that you’ll be discussing spoilers. In most cases it doesn’t even need to be spoiler-tagged out, except in cases of a major surprise point.