Why would it be a strong word? Here it is, in src/ls.c: https://github.com/coreutils/coreutils
It is then packaged by tens of operating system distributions, who themselves maintain extra patchsets, some of which are then upstreamed.
It is installed and used on millions (billions?) of devices, for 3 decades.
It's a very reliable and trusty "sharp stick of metal" :)
[0] https://www.cvedetails.com/vulnerability-list.php?vendor_id=...
No, the rely on POSIX[1][2]. Syscalls are an implementation detail handled by the compiler. Programs (like 'ls' and the rest of coreutils) that conform[3] to the POSIX standard should continue to work in future POSIX compatible operating systems.
If future operating systems deliberately introduce changes that break POSIX, then some work will be required to port the programs to the new environment. However, this type of porting process is really implementing a new feature. The original finished version isn't affected and shouldn't need maintenance.
[1] https://en.wikipedia.org/wiki/POSIX
[2] Other standards are also important, but I am only mentioning POSIX for brevity.
[3] https://github.com/coreutils/coreutils "Like the rest of the GNU system, these programs mostly conform to POSIX, with BSD and other extensions."
There will always be edge cases and scenarios we are not taking into account, please provide feedback on issue tracker and provide any suggestions so we can account these.