IIRC, that 14-byte limit stayed relevant for quite a while because ar (https://en.wikipedia.org/wiki/Ar_(Unix)#File_header), on some systems, was limited to 14-byte file names, even though the default file system allowed way longer names.

I remember hitting that problem on HP-UX while building gcc and GNU tools.

Aside: https://en.wikipedia.org/wiki/Ar_(Unix) says both “The ar format has never been standardized” and “Depending on the format, many ar implementations include a global symbol table (aka armap, directory or index) for fast linking without needing to scan the whole archive for a symbol. POSIX recognizes this feature, and requires ar implementations to have an -s option for updating it.”

Does that mean POSIX defines what the CLI ar tool must do, but not what the files it writes must look like?