It isn’t specifically spelled out in the Fortran standard. (Any of them.) But pretty much every compiler environment I used up until gfortran supported it.
A very long time ago, Fortran 66 days, I was helping develop a package which had a large library, and a bunch of programs which linked to it. The systems back then didn’t support shared libs. So using the external trick was important.
Since Fortran 90 introduced modules, thereby obsoleting common blocks and block data, it really hasn’t been a problem that I’d imagine the Committee have given any thought to. Especially since Fortran 2018…