Can you read Contributing Feature Requests and tell us the root problem you're having with Phabricator is? The tag isn't widely supported, so I can't see adding it.

Root of problem: if you create a big documents in wiki, reading this documents can be hard.

For resolve this problem we can add support for spoilers in editor and documents.

Yes, this standard supported by small count of browsers, but you can use jquery-details plugin, for example, to easy implement this function to all other browsers.

For example, look at this page, created with this tags :)

Every install that uses Phriction has long documents. We plan on adding additional features (like ToC improvements, builtin navigation, CMS, etc) to deal with this.

We don't have any interest in adding external dependencies like jquery, these represent a security risk we are unwilling to take into the upstream. If using a specific tag is all you want, you can implement a custom Remarkup rule locally. We haven't seen any interest in this specific solution and don't feel that hiding content is the best means of making Phriction easier to use.