At first I thought it was going to be some kind of solution to force all linux apps to adhere to the XDG Base Directory Specification, until I realized this related to a different specification altogether (XDG MIME Applications specification).

Making home read-only can get there, and I can't find the project that forced using XDG paths. I think it used LD_PRELOAD or somesuch trick.

edit: https://soc.me/standards/defending-home