pg_config() has been marked immutable since its inception. As part of a larger discussion around the definition of immutable versus stable and related implications for marking functions parallel safe raised by Andres, the consensus was clearly that pg_config() is stable, since it could possibly change output even for the same minor version with a recompile or installation of a new binary. So mark it stable.
Theoretically this could/should be backpatched, but it was deemed to be not worth the effort since in practice this is very unlikely to cause problems in the real world.
290e3b77fd Mark pg_config() stable rather than immutable
src/include/catalog/pg_proc.dat | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)