So all sorts of apps I run from the command line
spew this message:

Fontconfig warning: "/etc/fonts/conf.d/50-user.conf", line 9: reading configurations from ~/.fonts.conf is deprecated.

If it is gonna spew to the terminal anyway, wouldn't
it have been nice if the message said something about
where it now wants to read the config from?

After much googling, the algorithm is apparently:

mkdir .config/fontconfig
mv .fonts.conf .config/fontconfig/fonts.conf

The spewage went away after I did that.