Even more fun fact is that this # as EOL thing was introduced specially in XNU.

For the first second I thought this was in *BSD, but I did't found anything related in bsd source code (I was checking 4.3BSD and FreeBSD versions around 3.*)

Some blaming on xnu git[hub] mirror lead me to that.

This conceptual thing was introduced in XNU-517, about 21 years ago, and it's still in kernel.