I think you should just be careful and only 'spoil' when you've got something to say. So many times I've started reading a review and it's just a summery of the plot of the film, there's nothing in that, it's pointless.

More generally I think it's about context, you should be able to express your opinion about something without a dozen pop culture references, you should be able to say 'this is like what happens in X movie,' without having to spoil either piece of media. Also, while right now everything knows of something like the Thanos Snap, in 10 or 20 years there will be an entire audience of young people who don't.

Just be courteous, you don't know someone's situation or how important spoilers are to them.