On most Unix flavors it is not built-in. The only shell I know that has "ls" built-in is BusyBox.
And besides, I said in other places: "ls" has been "deprecated by its maintainer" more times than exa, it's just that somebody has always forked it. GNU "ls" (the one in Linux) is a complete rewrite of the original shell, and it is annoyingly incompatible with the macOS fork of BSD ls.
Worth noting that the Unix shell itself is not a "built-in". The kernel is agnostic about the user interface, which is left entirely to userspace programs to implement. (That was one of Unix's major innovations.)
> Applications should note that the standard PATH to the shell cannot be assumed to be either /bin/sh or /usr/bin/sh, and should be determined by interrogation of the PATH returned by getconf PATH, ensuring that the returned pathname is an absolute pathname and not a shell built-in.
And besides, I said in other places: "ls" has been "deprecated by its maintainer" more times than exa, it's just that somebody has always forked it. GNU "ls" (the one in Linux) is a complete rewrite of the original shell, and it is annoyingly incompatible with the macOS fork of BSD ls.