The Filesystem Hierarchy Standard is the standard around file/directory placement on Linux and Unix-like systems. Prominent open-source developer Matthias Klumpp announced today that the Filesystem Hierarchy Standard has been adopted by FreeDesktop.org. For the moment the FHS version is still 3.0, which was released a decade ago (May 2015).
Klumpp acknowledged in the announcement of adopting the FHS that it's "historically interesting" but in the future they hope this will lead to improvements/modernization.
The FHS specification can be found at specifications.freedesktop.org/fhs . The FHS specification itself is managed via this FreeDesktop.org GitLab repository for any modernization improvements and additions moving forward.