But indirectly, this functionality could exist in a Javascript widget/gadget/script that users who do not wish to see spoilers can install themselves, into their registered account's settings. The installation process is usually very simple, a small copy/paste into a configuration file.
I think that no direct modifications to articles to accommodate the script would be allowed; this includes special templates, <!-- hidden comments-->, or even the existing benign {{anchor}} template (usually used to mark HTML URL#link targets), if it's used to mark text {{anchor|spoiler}} for the script.
So the script has to be smart and dependent only the article text itself. It could selectively activate on articles in any category containing the words Television or Film, on sections named Plot or Synopsis, and simply collapse (autohide), or redact (blackout) any paragraphs which contain proper names, or keywords present in a list (die, dead, death, kill, wed, wedd*, marry, married, etc.)
A good starting point would be the text selection and redaction functionality as shown in the ProseSize.js script, which also presents a "Page size" link in the left navigation bar and, when clicked, instantly styles all the eligible article text background in yellow. Our widget could just use black.
I leave it to Javascript and CSS wizards to go further.