There is no the bash completion directory; there is a default one which happens to be system wide and only root writable (as should be the case in a combination

There is no the bash completion directory; there is a default one which happens to be system wide and only root writable (as should be the case in a combination of such circumstances). Fish allows for custom ones in ~/.config/fish and there is zero reason you cannot install custom ones in ~/.bashrc or user writable (on macOS) /usr/local