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.