This is the official syntax and should always work.

Spoiler alert: it doesn't:-P. There's a bunch of old apps that never bothered to implement it, yet people still continue to use them anyway.

Sometimes you'll see NSFW tags to accomplish the spoiler effect, on posts anyway (doesn't work on comments beneath a post).

It's always a tension between the things that move forward quickly vs. those that remain glacially slow or even half altogether. Like PieFed even has a separate NSFL (gore) tag to distinguish it from NSFW, and numerous options to either hide, blur, show as semi-transparent, etc., and you can pick a different rendering for each (one for NSFW but a different one for NSFL). And this on top of a plethora of features like polls, user and post flairs, hashtags, keyword filtering, user-customizable and shareable Feeds, and so much more. All of Lemmy remains behind in comparison. But even for Piefed, the vast majority of these features are not offered by the vast majority of apps available for the Threadiverse.

I suppose it's good to have choices. But that entails people making different ones. 😁