To clarify, I'm talking about blocking certain words with black rectangles, so that the user has to highlight the text in order to read it. The method I used to do this on older wikis doesn't seem to work anymore, so how do I do this now?
But do beware that if the page is filled whole-ly by spoiler text, it may violate the content policy of blocking and/or censoring content, or disrupting the content of pages
No, that is appropriate, you could also make a CSS code so when people hover their cursor over the spoiler text, the text will be visible. Put the code below in the wiki's CSS