I've known this since the start of my using unix shells, but it's almost useless. For example, if fzf or whichever plugin I'm using shows me the number, why would I need it when I'm going to use the list it has presented? I certainly wouldn't cancel it and use the number, nor would I type the number when I have the choice of banging one key repeatedly, or adding characters I can see in the command to narrow the search even more.
Some features just aren't worth the time.