Reviews in newspapers and magazines might OMIT key items and outcomes so as not to ruin plot twists, but they never put up spoiler warnings for the details they give away.
In other words: these sources are aware that people don't want to see spoilers, and take measures (omitting key items/outcomes) to keep spoilers away from such people. But since the measures they use aren't appropriate for us, we should use nothing at all.
If anything, the fact that newspapers and magazines recognize that spoilers are a problem should suggest that we should too.
(Besides, I've seen plenty of print reviews with spoiler warnings in them.)