No, it wouldn't!
First of all, while I don't have as narrow a view about tagging as SE seems to have and don't value the "meta tag" term they coined as much as other people, this tag would clearly be a "meta tag". It doesn't say anything about the question asked and merely says "it could contain spoilers". But you know what, that's what the whole site already says. A question about Jurassic World and its story does naturally contain spoilers about the movie. So when the question has the tag
jurassic-world, you most probably run the risk of spoilers in it (even more so if it has the
Which brings us to another important problem. It is unclear when to use that tag at all. First of all, what is a spoiler is absolutely not clearly definable in any way and is completely up to subjective opinion. But even worse, you want to use it for the purpose of directly marking spoilers, but for this purpose it is completely useless, since such a tag is bound to be used entirely incoherently and inconsistently. There is absolutely no guarantee that a question without that tag does not contain any spoilers, nor that a question with that tag does contain any spoilers. This makes its use lose any practical value and robs it of the single purpose you want to introduce it for.