[Fontconfig] Who is relying on the cache format on disk?

[L. A. Walsh]

Other than fontconfig programs & libs, what other programs are
relying on the exact format of the cache files on disk?

I.e. why is the internal format of the cache files, on disk, part
of the public API?   Who (what other projects) would be
affected if the on-disk cache format changed?

It seems like the internal format of the cache files isn't
used outside of the font-config library.  If that's the case,
then there is no cost to changing that format.

Who needs the cache format on disk NOT to change?