Users currently have special protected for their personal `.js` and `.css` pages. The protection entails that only they can create or edit them, with the exception of privileged users holding the `edituserjs` or `editusercss` user right.
There are even tools (Toolforge tools, Gadgets, user scripts, standalone apps) that encourage or require the storing of non-js content on a specially named `.js` sub page.
This use case is blocking T76204, which wants to give users a way to automatically validate their user scripts against common errors.
Per T76204#802378, and more generally, we should provide users with the option to store data (in the JSON format) under their user space with the same protection level as `.js` and `.css` pages currently receive.
In addition to being able to unblock T76204, by providing tools with a way to migrate their data, it is also more generally a useful feature for storing personal data. Some of which could be used in the future for storing other kinds of site customisations.